linux-kernel - Re: [PATCH bpf-next v1 4/4] resolve_btfids: change in-place update with raw binary output

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Message-ID: <9ac1ab7b-1412-4e81-a993-df95c372c4d8@linux.dev>
Date: Wed, 26 Nov 2025 15:58:01 -0800
From: Ihor Solodrai <ihor.solodrai@...ux.dev>
To: Donglin Peng <dolinux.peng@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman
 <eddyz87@...il.com>, Song Liu <song@...nel.org>,
 Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, Nathan Chancellor <nathan@...nel.org>,
 Nicolas Schier <nicolas.schier@...ux.dev>,
 Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
 bpf@...r.kernel.org, dwarves@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-kbuild@...r.kernel.org, Alan Maguire <alan.maguire@...cle.com>
Subject: Re: [PATCH bpf-next v1 4/4] resolve_btfids: change in-place update
 with raw binary output

On 11/26/25 11:13 AM, Ihor Solodrai wrote:
> On 11/26/25 5:03 AM, Donglin Peng wrote:
>> On Wed, Nov 26, 2025 at 9:29 AM Ihor Solodrai <ihor.solodrai@...ux.dev> wrote:
>>>
>>> [...]
>>>
>>> For the kernel modules creating special .bpf.o file is not necessary,
>>> and so embedding of sections data produced by resolve_btfids is
>>> straightforward with the objcopy.
>>
>> The Makefile for the bpf selftests also needs be updated too:
>> https://elixir.bootlin.com/linux/v6.18-rc7/source/tools/testing/selftests/bpf/Makefile#L708
>>
>> This results in the self-test for resolve_btfids failing:
>>  $./vmtest.sh -- ./test_progs -t resolve_btfids -v
>> ...
>> test_resolve_btfids:PASS:id_check 0 nsec
>> test_resolve_btfids:FAIL:id_check wrong ID for S (0 != 3)
> 
> Good catch, thanks.
> 
> I remember I noticed this at some point, and then forgot...
> 
> Interestingly this test passes on CI [1]: 
> 
> 2025-11-26T03:09:52.0908317Z #366     reg_bounds_rand_ranges_u64_u64:OK
> 2025-11-26T03:09:52.0925114Z #367     resolve_btfids:OK
> 2025-11-26T03:09:52.3904190Z #368/1   res_spin_lock_failure/res_spin_lock_arg:OK
> 
> I'll take a closer look.

I figured out why this test was flaky.

Even though I removed elf_update() call from resolve_btfids, the ELF
was opened with:

    elf = elf_begin(fd, ELF_C_RDWR_MMAP, NULL);

And the buffers which resolve_btfids writes to are from Elf_Data
returned by elf_getdata(). And so the file might actually get written
to in-place, which is why the resolve_btfids test passed for me with
no changes to the selftests.

I switched ELF_C_RDWR_MMAP to ELF_C_READ_MMAP_PRIVATE, and then the
ELF reliably remains intact (and the test fails). From libelf.h:

  ELF_C_READ_MMAP_PRIVATE,	/* Read, but memory is writable, results are
				   not written to the file.  */

It makes sense to use this for what resolve_btfids is doing.

I'll fix selftests/bpf/Makefile in the next revision.


> 
> [1] https://github.com/kernel-patches/bpf/actions/runs/19690981192/job/56406840021
> 
>>
>>
>>> [...]
>>>
>