On 2023-05-12, Kees Cook wrote:
On Fri, May 12, 2023 at 02:25:28AM +0000, Fangrui Song wrote:
Tools like readelf/llvm-readelf use p_align to parse a PT_NOTE program
header as an array of 4-byte entries or 8-byte entries. Currently, there
are workarounds[1] in place for Linux to treat p_align==0 as 4. However,
it would be more appropriate to set the correct alignment so that tools
do not have to rely on guesswork. FreeBSD coredumps set p_align to 4 as
well.
[1]: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=82ed9683ec099d8205dc499ac84febc975235af6
The interesting bit from here is:
/* NB: Some note sections may have alignment value of 0 or 1. gABI
specifies that notes should be aligned to 4 bytes in 32-bit
objects and to 8 bytes in 64-bit objects. As a Linux extension,
we also support 4 byte alignment in 64-bit objects. If section
alignment is less than 4, we treate alignment as 4 bytes. */
if (align < 4)
align = 4;
else if (align != 4 && align != 8)
{
warn (_("Corrupt note: alignment %ld, expecting 4 or 8\n"),
(long) align);
return FALSE;
}
Should Linux use 8 for 64-bit processes to avoid the other special case?
(And do we need to make some changes to make sure we are actually
aligned?)
-Kees
64-bit objects should use 8-byte entries and naturally the 8-byte alignment.
Unfortunately, many systems including Solaris, *BSD, and Linux use
4-byte entries for SHT_NOTE/PT_NOTE, and changing this will create
a large compatibility problem (see tcmalloc that I recently
updated[1])
Linux introduced 8-byte alignment note sections (.note.gnu.property) a
while ago, so the ecosystem has to deal with notes of mixed alignments.
The resolution is to use the note alignment to decide whether it should
be parsed as 4-byte entries or 8-byte entries.
I think that just setting `p_align = 4` on the kernel side should be
good enough:)
[1]:
https://github.com/google/tcmalloc/commit/c33cb2d8935002f8ba942028a1f0871d075345a1
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