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physmem: fix qemu_ram_alloc_from_fd size calculation

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qemu_ram_alloc_from_fd allocates space if file_size == 0.  If non-zero,
it uses the existing space and verifies it is large enough, but the
verification was broken when the offset parameter was introduced.  As
a result, a file smaller than offset passes the verification and causes
errors later.  Fix that, and update the error message to include offset.

Peter provides this concise reproducer:

  $ touch ramfile
  $ truncate -s 64M ramfile
  $ ./qemu-system-x86_64 -object memory-backend-file,mem-path=./ramfile,offset=128M,size=128M,id=mem1,prealloc=on
  qemu-system-x86_64: qemu_prealloc_mem: preallocating memory failed: Bad address

With the fix, the error message is:
  qemu-system-x86_64: mem1 backing store size 0x4000000 is too small for 'size' option 0x8000000 plus 'offset' option 0x8000000

Cc: qemu-stable@nongnu.org
Fixes: 4b870dc4d0c0 ("hostmem-file: add offset option")
Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/1736967650-129648-3-git-send-email-steven.sistare@oracle.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>
This commit is contained in:
Steve Sistare 2025-01-15 11:00:28 -08:00 committed by Fabiano Rosas
parent 57ad6ab804
commit 719168fba7
1 changed files with 6 additions and 4 deletions
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10
system/physmem.c
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@ -1970,10 +1970,12 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
size = REAL_HOST_PAGE_ALIGN(size);
file_size = get_file_size(fd);
if (file_size > offset && file_size < (offset + size)) {
error_setg(errp, "backing store size 0x%" PRIx64
" does not match 'size' option 0x" RAM_ADDR_FMT,
file_size, size);
if (file_size && file_size < offset + size) {
error_setg(errp, "%s backing store size 0x%" PRIx64
" is too small for 'size' option 0x" RAM_ADDR_FMT
" plus 'offset' option 0x%" PRIx64,
memory_region_name(mr), file_size, size,
(uint64_t)offset);
return NULL;
}