Messages in this thread | | | Date | Thu, 19 Aug 2021 16:24:37 -0700 | From | Josh Poimboeuf <> | Subject | Re: [PATCH] objtool: x86: .altinstructions doesn't need section entry size |
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On Mon, Aug 16, 2021 at 09:49:58PM -0400, Joe Lawrence wrote: > commit e31694e0a7a7 ("objtool: Don't make .altinstructions writable") > aligned objtool-created and kernel-created .altinstructions section > flags, but there remains a minor discrepency in their use of a section > entry size: objtool sets one while the kernel build does not.
I'd recommend more of an "active voice" subject like:
objtool: Make .altinstructions section entry size consistent
> > While sh_entsize of sizeof(struct alt_instr) seems intuitive, this small > deviation can cause failures with external tooling like kpatch-build. > > Fix this by creating new .altinstructions sections with sh_entsize of 0 > and then later updating sec->len as alternatives are added to the > section. An added benefit is avoiding the data descriptor and buffer > created by elf_create_section(), but previously unused by > elf_add_alternative(). > > Fixes: 9bc0bb50727c ("objtool/x86: Rewrite retpoline thunk calls") > Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com> > --- > > Hi Josh, this is a follow up for > https://github.com/dynup/kpatch/issues/1194 where I'll add some more > comments on the kpatch-side of this. We could probably work around it > over there, but this objtool tweak looks small enough to maintain closer > kernel-built .altinstructions section properties. > > tools/objtool/arch/x86/decode.c | 4 +++- > 1 file changed, 3 insertions(+), 1 deletion(-) > > diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c > index bc821056aba9..e7087aa473f8 100644 > --- a/tools/objtool/arch/x86/decode.c > +++ b/tools/objtool/arch/x86/decode.c > @@ -684,7 +684,7 @@ static int elf_add_alternative(struct elf *elf, > sec = find_section_by_name(elf, ".altinstructions"); > if (!sec) { > sec = elf_create_section(elf, ".altinstructions", > - SHF_ALLOC, size, 0); > + SHF_ALLOC, 0, 0);
Looks good.
> > if (!sec) { > WARN_ELF("elf_create_section"); > @@ -692,6 +692,8 @@ static int elf_add_alternative(struct elf *elf, > } > } > > + sec->len += size; > +
This latter change makes sense, but I'm not sure it belongs in this patch. Wasn't sec->len wrongly set to zero (and never incremented) even before this patch?
From what I can tell sec->len isn't ever read for this section, so it seems to be more of a previously existing theoretical bug, independent of the entsize mismatch bug. In which case it's still worth fixing, just probably in a separate patch.
Also the sec->len update should probably be moved down to the bottom of the function alongside the update to sec->sh.sh_size, as sec->len is a "convenient" more readable mirror copy of sec->sh.sh_size.
Actually, mirroring sec->sh.sh_size was a bad idea. It's guaranteed to introduce dumb bugs like this. Maybe we should just kill sec->len altogether in favor of using sec->sh.sh_size everywhere.
-- Josh
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