linux-kernel - Re: [PATCH] vmlinux.lds.h: Force-align ELF notes section to four bytes

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Date:   Sun, 16 Apr 2023 18:52:04 +0000
From:   Joan Bruguera Micó <joanbrugueram@...il.com>
To:     jpoimboe@...nel.org, i.pear@...look.com
Cc:     acme@...nel.org, alan.maguire@...cle.com, alexandref75@...il.com,
        bpf@...r.kernel.org, dxu@...uu.xyz, jforbes@...hat.com,
        linux-kernel@...r.kernel.org, olsajiri@...il.com,
        peterz@...radead.org, ptalbert@...hat.com, yhs@...com
Subject: Re: [PATCH] vmlinux.lds.h: Force-align ELF notes section to four bytes

On Thu, Apr 13, 2023 at 12:54:19PM +0800, Tianyi Liu wrote:
> On Wed, Apr 12, 2023 at 16:30PM UTC, Josh Poimboeuf wrote:
> > On Wed, Apr 12, 2023 at 03:10:14PM +0800, Tianyi Liu wrote:
> > > On Tue, Apr 11, 2023 at 17:00 , Josh Poimboeuf wrote:
> > > > On Tue, Feb 14, 2023 at 02:33:02PM +0800, Tianyi Liu wrote:
> > > > > > LLVM_OBJCOPY=objcopy pahole -J --btf_gen_floats -j
> > > > > > --skip_encoding_btf_inconsistent_proto --btf_gen_optimized
> > > > > > .tmp_vmlinux.btf
> > > > > > btf_encoder__encode: btf__dedup failed!
> > > > > > Failed to encode BTF
> > > > > >
> > > > > > Thanks,
> > > > > >
> > > > >
> > > > > I encountered the same problem when building a new kernel and I found some
> > > > > reasons for the error.
> > > > >
> > > > > In short, enabling CONFIG_X86_KERNEL_IBT will change the order of records in
> > > > > .notes section. In addition, due to historical problems, the alignment of
> > > > > records in the .notes section is not unified, which leads to the inability of
> > > > > gelf_getnote() to read the records after the wrong one.
> > > >
> > > > Alexandre, Tianyi, are you still seeing this issue with the latest
> > > > dwarves?  If so can you confirm the below patch fixes it?
> > > >
> > >
> > > Josh, first of all, thank you very much for your help. However, this patch
> > > doesn't work in my environment. I am using gcc 12.2.1 20230201.
> > > After compiling, when I use readelf -S to view ELF sections,
> > > the align of .notes section is still 8:
> > >
> > > $ readelf -S .tmp_vmlinux.btf
> > > [20] .notes            NOTE             ffffffff8250b570  0170b570
> > >      0000000000000084  0000000000000000   A       0     0     8
> > 
> > Hm, weird.
> 
> I have consulted some materials and found that using ALIGN() in linker
> scripts can only "increase" alignment, not decrease it.
> 
> Perhaps could you try modifying your patch to use ALIGN(2) and SUBALIGN(2)
> and see if the .notes section in the output file is aligned to 2?
> In my tests, this had no effect.
> 
> [1] https://sourceware.org/binutils/docs/ld/Forced-Output-Alignment.html

Not sure about the ld documentation (it may just be ambiguous wording)
but while doing some tests, I have found that Josh's ALIGN/SUBALIGN(4)
patch only works for certain kernel configs and fails for others.
This likely explains why Josh's patch worked for me but not for Tianyi.

I haven't investigated why or when it works though, but here's a config
where Josh's ALIGN(4) patch works and or where it doesn't (for me):
https://zealcharm.com/20230416-btf-dedup-bug-sample-configs/align-4-working
https://zealcharm.com/20230416-btf-dedup-bug-sample-configs/align-4-not-working

OTOH the new patch to discard .note.gnu.property works in both cases.