470 lines
17 KiB
ReStructuredText
470 lines
17 KiB
ReStructuredText
=================
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SanitizerCoverage
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=================
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.. contents::
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:local:
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Introduction
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============
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LLVM has a simple code coverage instrumentation built in (SanitizerCoverage).
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It inserts calls to user-defined functions on function-, basic-block-, and edge- levels.
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Default implementations of those callbacks are provided and implement
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simple coverage reporting and visualization,
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however if you need *just* coverage visualization you may want to use
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:doc:`SourceBasedCodeCoverage <SourceBasedCodeCoverage>` instead.
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Tracing PCs with guards
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=======================
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With ``-fsanitize-coverage=trace-pc-guard`` the compiler will insert the following code
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on every edge:
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.. code-block:: none
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__sanitizer_cov_trace_pc_guard(&guard_variable)
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Every edge will have its own `guard_variable` (uint32_t).
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The compiler will also insert calls to a module constructor:
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.. code-block:: c++
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// The guards are [start, stop).
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// This function will be called at least once per DSO and may be called
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// more than once with the same values of start/stop.
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__sanitizer_cov_trace_pc_guard_init(uint32_t *start, uint32_t *stop);
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With an additional ``...=trace-pc,indirect-calls`` flag
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``__sanitizer_cov_trace_pc_indirect(void *callee)`` will be inserted on every indirect call.
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The functions `__sanitizer_cov_trace_pc_*` should be defined by the user.
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Example:
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.. code-block:: c++
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// trace-pc-guard-cb.cc
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#include <stdint.h>
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#include <stdio.h>
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#include <sanitizer/coverage_interface.h>
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// This callback is inserted by the compiler as a module constructor
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// into every DSO. 'start' and 'stop' correspond to the
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// beginning and end of the section with the guards for the entire
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// binary (executable or DSO). The callback will be called at least
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// once per DSO and may be called multiple times with the same parameters.
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extern "C" void __sanitizer_cov_trace_pc_guard_init(uint32_t *start,
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uint32_t *stop) {
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static uint64_t N; // Counter for the guards.
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if (start == stop || *start) return; // Initialize only once.
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printf("INIT: %p %p\n", start, stop);
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for (uint32_t *x = start; x < stop; x++)
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*x = ++N; // Guards should start from 1.
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}
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// This callback is inserted by the compiler on every edge in the
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// control flow (some optimizations apply).
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// Typically, the compiler will emit the code like this:
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// if(*guard)
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// __sanitizer_cov_trace_pc_guard(guard);
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// But for large functions it will emit a simple call:
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// __sanitizer_cov_trace_pc_guard(guard);
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extern "C" void __sanitizer_cov_trace_pc_guard(uint32_t *guard) {
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if (!*guard) return; // Duplicate the guard check.
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// If you set *guard to 0 this code will not be called again for this edge.
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// Now you can get the PC and do whatever you want:
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// store it somewhere or symbolize it and print right away.
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// The values of `*guard` are as you set them in
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// __sanitizer_cov_trace_pc_guard_init and so you can make them consecutive
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// and use them to dereference an array or a bit vector.
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void *PC = __builtin_return_address(0);
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char PcDescr[1024];
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// This function is a part of the sanitizer run-time.
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// To use it, link with AddressSanitizer or other sanitizer.
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__sanitizer_symbolize_pc(PC, "%p %F %L", PcDescr, sizeof(PcDescr));
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printf("guard: %p %x PC %s\n", guard, *guard, PcDescr);
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}
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.. code-block:: c++
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// trace-pc-guard-example.cc
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void foo() { }
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int main(int argc, char **argv) {
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if (argc > 1) foo();
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}
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.. code-block:: console
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clang++ -g -fsanitize-coverage=trace-pc-guard trace-pc-guard-example.cc -c
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clang++ trace-pc-guard-cb.cc trace-pc-guard-example.o -fsanitize=address
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ASAN_OPTIONS=strip_path_prefix=`pwd`/ ./a.out
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.. code-block:: console
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INIT: 0x71bcd0 0x71bce0
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guard: 0x71bcd4 2 PC 0x4ecd5b in main trace-pc-guard-example.cc:2
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guard: 0x71bcd8 3 PC 0x4ecd9e in main trace-pc-guard-example.cc:3:7
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.. code-block:: console
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ASAN_OPTIONS=strip_path_prefix=`pwd`/ ./a.out with-foo
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.. code-block:: console
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INIT: 0x71bcd0 0x71bce0
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guard: 0x71bcd4 2 PC 0x4ecd5b in main trace-pc-guard-example.cc:3
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guard: 0x71bcdc 4 PC 0x4ecdc7 in main trace-pc-guard-example.cc:4:17
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guard: 0x71bcd0 1 PC 0x4ecd20 in foo() trace-pc-guard-example.cc:2:14
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Inline 8bit-counters
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====================
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**Experimental, may change or disappear in future**
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With ``-fsanitize-coverage=inline-8bit-counters`` the compiler will insert
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inline counter increments on every edge.
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This is similar to ``-fsanitize-coverage=trace-pc-guard`` but instead of a
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callback the instrumentation simply increments a counter.
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Users need to implement a single function to capture the counters at startup.
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.. code-block:: c++
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extern "C"
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void __sanitizer_cov_8bit_counters_init(char *start, char *end) {
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// [start,end) is the array of 8-bit counters created for the current DSO.
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// Capture this array in order to read/modify the counters.
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}
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Inline bool-flag
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================
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**Experimental, may change or disappear in future**
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With ``-fsanitize-coverage=inline-bool-flag`` the compiler will insert
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setting an inline boolean to true on every edge.
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This is similar to ``-fsanitize-coverage=inline-8bit-counter`` but instead of
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an increment of a counter, it just sets a boolean to true.
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Users need to implement a single function to capture the flags at startup.
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.. code-block:: c++
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extern "C"
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void __sanitizer_cov_bool_flag_init(bool *start, bool *end) {
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// [start,end) is the array of boolean flags created for the current DSO.
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// Capture this array in order to read/modify the flags.
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}
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PC-Table
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========
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**Experimental, may change or disappear in future**
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**Note:** this instrumentation might be incompatible with dead code stripping
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(``-Wl,-gc-sections``) for linkers other than LLD, thus resulting in a
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significant binary size overhead. For more information, see
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`Bug 34636 <https://bugs.llvm.org/show_bug.cgi?id=34636>`_.
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With ``-fsanitize-coverage=pc-table`` the compiler will create a table of
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instrumented PCs. Requires either ``-fsanitize-coverage=inline-8bit-counters``,
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or ``-fsanitize-coverage=inline-bool-flag``, or ``-fsanitize-coverage=trace-pc-guard``.
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Users need to implement a single function to capture the PC table at startup:
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.. code-block:: c++
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extern "C"
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void __sanitizer_cov_pcs_init(const uintptr_t *pcs_beg,
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const uintptr_t *pcs_end) {
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// [pcs_beg,pcs_end) is the array of ptr-sized integers representing
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// pairs [PC,PCFlags] for every instrumented block in the current DSO.
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// Capture this array in order to read the PCs and their Flags.
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// The number of PCs and PCFlags for a given DSO is the same as the number
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// of 8-bit counters (-fsanitize-coverage=inline-8bit-counters), or
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// boolean flags (-fsanitize-coverage=inline=bool-flags), or trace_pc_guard
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// callbacks (-fsanitize-coverage=trace-pc-guard).
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// A PCFlags describes the basic block:
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// * bit0: 1 if the block is the function entry block, 0 otherwise.
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}
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Tracing PCs
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===========
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With ``-fsanitize-coverage=trace-pc`` the compiler will insert
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``__sanitizer_cov_trace_pc()`` on every edge.
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With an additional ``...=trace-pc,indirect-calls`` flag
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``__sanitizer_cov_trace_pc_indirect(void *callee)`` will be inserted on every indirect call.
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These callbacks are not implemented in the Sanitizer run-time and should be defined
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by the user.
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This mechanism is used for fuzzing the Linux kernel
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(https://github.com/google/syzkaller).
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Instrumentation points
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======================
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Sanitizer Coverage offers different levels of instrumentation.
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* ``edge`` (default): edges are instrumented (see below).
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* ``bb``: basic blocks are instrumented.
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* ``func``: only the entry block of every function will be instrumented.
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Use these flags together with ``trace-pc-guard`` or ``trace-pc``,
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like this: ``-fsanitize-coverage=func,trace-pc-guard``.
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When ``edge`` or ``bb`` is used, some of the edges/blocks may still be left
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uninstrumented (pruned) if such instrumentation is considered redundant.
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Use ``no-prune`` (e.g. ``-fsanitize-coverage=bb,no-prune,trace-pc-guard``)
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to disable pruning. This could be useful for better coverage visualization.
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Edge coverage
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-------------
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Consider this code:
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.. code-block:: c++
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void foo(int *a) {
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if (a)
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*a = 0;
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}
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It contains 3 basic blocks, let's name them A, B, C:
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.. code-block:: none
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A
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C
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If blocks A, B, and C are all covered we know for certain that the edges A=>B
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and B=>C were executed, but we still don't know if the edge A=>C was executed.
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Such edges of control flow graph are called
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`critical <https://en.wikipedia.org/wiki/Control_flow_graph#Special_edges>`_.
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The edge-level coverage simply splits all critical edges by introducing new
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dummy blocks and then instruments those blocks:
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.. code-block:: none
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A
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D B
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C
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Tracing data flow
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=================
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Support for data-flow-guided fuzzing.
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With ``-fsanitize-coverage=trace-cmp`` the compiler will insert extra instrumentation
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around comparison instructions and switch statements.
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Similarly, with ``-fsanitize-coverage=trace-div`` the compiler will instrument
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integer division instructions (to capture the right argument of division)
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and with ``-fsanitize-coverage=trace-gep`` --
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the `LLVM GEP instructions <https://llvm.org/docs/GetElementPtr.html>`_
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(to capture array indices).
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Unless ``no-prune`` option is provided, some of the comparison instructions
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will not be instrumented.
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.. code-block:: c++
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// Called before a comparison instruction.
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// Arg1 and Arg2 are arguments of the comparison.
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void __sanitizer_cov_trace_cmp1(uint8_t Arg1, uint8_t Arg2);
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void __sanitizer_cov_trace_cmp2(uint16_t Arg1, uint16_t Arg2);
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void __sanitizer_cov_trace_cmp4(uint32_t Arg1, uint32_t Arg2);
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void __sanitizer_cov_trace_cmp8(uint64_t Arg1, uint64_t Arg2);
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// Called before a comparison instruction if exactly one of the arguments is constant.
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// Arg1 and Arg2 are arguments of the comparison, Arg1 is a compile-time constant.
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// These callbacks are emitted by -fsanitize-coverage=trace-cmp since 2017-08-11
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void __sanitizer_cov_trace_const_cmp1(uint8_t Arg1, uint8_t Arg2);
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void __sanitizer_cov_trace_const_cmp2(uint16_t Arg1, uint16_t Arg2);
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void __sanitizer_cov_trace_const_cmp4(uint32_t Arg1, uint32_t Arg2);
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void __sanitizer_cov_trace_const_cmp8(uint64_t Arg1, uint64_t Arg2);
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// Called before a switch statement.
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// Val is the switch operand.
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// Cases[0] is the number of case constants.
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// Cases[1] is the size of Val in bits.
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// Cases[2:] are the case constants.
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void __sanitizer_cov_trace_switch(uint64_t Val, uint64_t *Cases);
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// Called before a division statement.
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// Val is the second argument of division.
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void __sanitizer_cov_trace_div4(uint32_t Val);
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void __sanitizer_cov_trace_div8(uint64_t Val);
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// Called before a GetElemementPtr (GEP) instruction
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// for every non-constant array index.
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void __sanitizer_cov_trace_gep(uintptr_t Idx);
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Partially disabling instrumentation
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===================================
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It is sometimes useful to tell SanitizerCoverage to instrument only a subset of the
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functions in your target.
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With ``-fsanitize-coverage-allowlist=allowlist.txt``
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and ``-fsanitize-coverage-blocklist=blocklist.txt``,
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you can specify such a subset through the combination of a allowlist and a blocklist.
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SanitizerCoverage will only instrument functions that satisfy two conditions.
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First, the function should belong to a source file with a path that is both allowlisted
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and not blocklisted.
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Second, the function should have a mangled name that is both allowlisted and not blocklisted.
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The allowlist and blocklist format is similar to that of the sanitizer blocklist format.
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The default allowlist will match every source file and every function.
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The default blocklist will match no source file and no function.
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A common use case is to have the allowlist list folders or source files for which you want
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instrumentation and allow all function names, while the blocklist will opt out some specific
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files or functions that the allowlist loosely allowed.
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Here is an example allowlist:
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.. code-block:: none
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# Enable instrumentation for a whole folder
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src:bar/*
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# Enable instrumentation for a specific source file
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src:foo/a.cpp
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# Enable instrumentation for all functions in those files
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fun:*
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And an example blocklist:
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.. code-block:: none
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# Disable instrumentation for a specific source file that the allowlist allowed
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src:bar/b.cpp
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# Disable instrumentation for a specific function that the allowlist allowed
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fun:*myFunc*
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The use of ``*`` wildcards above is required because function names are matched after mangling.
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Without the wildcards, one would have to write the whole mangled name.
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Be careful that the paths of source files are matched exactly as they are provided on the clang
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command line.
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For example, the allowlist above would include file ``bar/b.cpp`` if the path was provided
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exactly like this, but would it would fail to include it with other ways to refer to the same
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file such as ``./bar/b.cpp``, or ``bar\b.cpp`` on Windows.
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So, please make sure to always double check that your lists are correctly applied.
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Default implementation
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======================
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The sanitizer run-time (AddressSanitizer, MemorySanitizer, etc) provide a
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default implementations of some of the coverage callbacks.
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You may use this implementation to dump the coverage on disk at the process
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exit.
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Example:
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.. code-block:: console
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% cat -n cov.cc
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1 #include <stdio.h>
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2 __attribute__((noinline))
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3 void foo() { printf("foo\n"); }
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4
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5 int main(int argc, char **argv) {
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6 if (argc == 2)
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7 foo();
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8 printf("main\n");
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9 }
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% clang++ -g cov.cc -fsanitize=address -fsanitize-coverage=trace-pc-guard
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% ASAN_OPTIONS=coverage=1 ./a.out; wc -c *.sancov
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main
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SanitizerCoverage: ./a.out.7312.sancov 2 PCs written
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24 a.out.7312.sancov
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% ASAN_OPTIONS=coverage=1 ./a.out foo ; wc -c *.sancov
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foo
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main
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SanitizerCoverage: ./a.out.7316.sancov 3 PCs written
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24 a.out.7312.sancov
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32 a.out.7316.sancov
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Every time you run an executable instrumented with SanitizerCoverage
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one ``*.sancov`` file is created during the process shutdown.
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If the executable is dynamically linked against instrumented DSOs,
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one ``*.sancov`` file will be also created for every DSO.
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Sancov data format
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------------------
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The format of ``*.sancov`` files is very simple: the first 8 bytes is the magic,
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one of ``0xC0BFFFFFFFFFFF64`` and ``0xC0BFFFFFFFFFFF32``. The last byte of the
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magic defines the size of the following offsets. The rest of the data is the
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offsets in the corresponding binary/DSO that were executed during the run.
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Sancov Tool
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-----------
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An simple ``sancov`` tool is provided to process coverage files.
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The tool is part of LLVM project and is currently supported only on Linux.
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It can handle symbolization tasks autonomously without any extra support
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from the environment. You need to pass .sancov files (named
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``<module_name>.<pid>.sancov`` and paths to all corresponding binary elf files.
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Sancov matches these files using module names and binaries file names.
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.. code-block:: console
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USAGE: sancov [options] <action> (<binary file>|<.sancov file>)...
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Action (required)
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-print - Print coverage addresses
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-covered-functions - Print all covered functions.
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-not-covered-functions - Print all not covered functions.
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-symbolize - Symbolizes the report.
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Options
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-blocklist=<string> - Blocklist file (sanitizer blocklist format).
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-demangle - Print demangled function name.
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-strip_path_prefix=<string> - Strip this prefix from file paths in reports
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Coverage Reports
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----------------
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**Experimental**
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``.sancov`` files do not contain enough information to generate a source-level
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coverage report. The missing information is contained
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in debug info of the binary. Thus the ``.sancov`` has to be symbolized
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to produce a ``.symcov`` file first:
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.. code-block:: console
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sancov -symbolize my_program.123.sancov my_program > my_program.123.symcov
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The ``.symcov`` file can be browsed overlaid over the source code by
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running ``tools/sancov/coverage-report-server.py`` script that will start
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an HTTP server.
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Output directory
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----------------
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By default, .sancov files are created in the current working directory.
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This can be changed with ``ASAN_OPTIONS=coverage_dir=/path``:
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.. code-block:: console
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% ASAN_OPTIONS="coverage=1:coverage_dir=/tmp/cov" ./a.out foo
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% ls -l /tmp/cov/*sancov
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-rw-r----- 1 kcc eng 4 Nov 27 12:21 a.out.22673.sancov
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-rw-r----- 1 kcc eng 8 Nov 27 12:21 a.out.22679.sancov
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