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What to do when sh_addralign isn't a power of two

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BFD generally doesn't handle anything but a power of two section
alignment, and ELF sh_addralign is required to be an integral power of
two (or zero) by the ELF spec.  Of course this is ignored by fuzzers,
and because bfd_log2 rounds up, we can end up with alignment_power
being 32 on a 32-bit object or 64 on a 64-bit object.  That then
triggers ubsan warnings in places like bfd_update_compression_header
where we want to convert from alignment_power back to an alignment.
I suppose we could reject object files that have non-compliant
sh_addralign, but I think it's also reasonable to use the greatest
power of two divisor of sh_addralign, ie. the rightmost 1 bit.

	* elf.c (_bfd_elf_make_section_from_shdr): Use greatest power
	of two divisor of sh_addralign.
	(_bfd_elf_assign_file_position_for_section): Likewise.
	(assign_file_positions_for_non_load_sections): Likewise.
This commit is contained in:
Alan Modra
2022-02-15 20:11:03 +10:30
parent 6e73172988
commit 1f9b1a8435
1 changed files with 7 additions and 6 deletions
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13
bfd/elf.c
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@@ -1104,7 +1104,8 @@ _bfd_elf_make_section_from_shdr (bfd *abfd,
if (!bfd_set_section_vma (newsect, hdr->sh_addr / opb)
|| !bfd_set_section_size (newsect, hdr->sh_size)
|| !bfd_set_section_alignment (newsect, bfd_log2 (hdr->sh_addralign)))
|| !bfd_set_section_alignment (newsect, bfd_log2 (hdr->sh_addralign
& -hdr->sh_addralign)))
return false;
/* As a GNU extension, if the name begins with .gnu.linkonce, we
@@ -4227,7 +4228,7 @@ _bfd_elf_assign_file_position_for_section (Elf_Internal_Shdr *i_shdrp,
bool align)
{
if (align && i_shdrp->sh_addralign > 1)
offset = BFD_ALIGN (offset, i_shdrp->sh_addralign);
offset = BFD_ALIGN (offset, i_shdrp->sh_addralign & -i_shdrp->sh_addralign);
i_shdrp->sh_offset = offset;
if (i_shdrp->bfd_section != NULL)
i_shdrp->bfd_section->filepos = offset;
@@ -6149,6 +6150,7 @@ assign_file_positions_for_non_load_sections (bfd *abfd,
for (hdrpp = i_shdrpp + 1; hdrpp < end_hdrpp; hdrpp++)
{
Elf_Internal_Shdr *hdr;
bfd_vma align;
hdr = *hdrpp;
if (hdr->bfd_section != NULL
@@ -6174,11 +6176,10 @@ assign_file_positions_for_non_load_sections (bfd *abfd,
: hdr->bfd_section->name));
/* We don't need to page align empty sections. */
if ((abfd->flags & D_PAGED) != 0 && hdr->sh_size != 0)
off += vma_page_aligned_bias (hdr->sh_addr, off,
maxpagesize);
align = maxpagesize;
else
off += vma_page_aligned_bias (hdr->sh_addr, off,
hdr->sh_addralign);
align = hdr->sh_addralign & -hdr->sh_addralign;
off += vma_page_aligned_bias (hdr->sh_addr, off, align);
off = _bfd_elf_assign_file_position_for_section (hdr, off,
false);
}