Re: [BUG] bpf: tcp: bpf_tcp_ops_set_rcvlowat() missing from BTF with clang KCSAN

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- [Low] bpf: tcp: missing Fixes trailer and truncated commit log
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commit f5456603019de81f20f04a8d77446db12563aeac
Author: Breno Leitao <leitao@debian.org>

> bpf: tcp: bpf_tcp_ops_set_rcvlowat() missing from BTF with clang KCSAN
>
> My arm64 KCSAN builds of linux-next fail since next-20261015, because
> bpf_tcp_ops_set_rcvlowat() is missing from the vmlinux BTF:
>
>   WARN: resolve_btfids: no BTF func for kfunc bpf_tcp_ops_set_rcvlowat in bpf_tcp_ops_set_rcvlowat_kfunc_set
>   WARN: resolve_btfids: unresolved symbol bpf_tcp_ops_set_rcvlowat
>   Failed to generate BTF for vmlinux
>   Try to disable CONFIG_DEBUG_INFO_BTF
>   make[4]: *** [scripts/Makefile.vmlinux:72: vmlinux.unstripped] Error 1
>
> This is clang 22.1.3 and pahole 1.31 (current pahole master behaves the
> same), with CONFIG_KCSAN=y, CONFIG_DEBUG_INFO_BTF=y and CONFIG_WERROR=y.
> v7.4-rc1 is affected as well. With pahole 1.31, the same config without
> KCSAN builds fine.
>
> The kfunc came with 5f549c1e49f3 ("bpf: tcp: Add kfunc to adjust
> sk->sk_rcvlowat."), and 201a454df948 ("resolve_btfids: Fix the
> --fatal_warnings exit status") made the missing BTF fatal with
> CONFIG_WERROR. Without CONFIG_WERROR the kernel builds, but bpf_tcp_ops
> is never registered:
>
>   initcall __bpf_tcp_ops_init+0x0/0x70 returned -22 after 302 usecs
>
> This seems similar to pahole behaviour sched_ext hit with a bool
> argument, discussed in [2] and worked around in f2f095c87603
> ("sched_ext: Work around pahole 1.32 dropping
> scx_bpf_task_set_lazy_resched() from BTF").
>
> Here the argument is an int that clang reassigns, its location list has
> no DW_OP_entry_value, and pahole 1.31 drops it as well, so staying on
> pahole 1.31 does not help.
>
> Not reassigning the parameter is a workaround for me, with any pahole: clang
> then describes @rcvlowat in w1 from the entry, the kfunc is back in BTF,
> and __bpf_tcp_ops_init() returns 0 (tested in virtme):

[Severity: Low]
This fixes a build and kfunc registration failure in
bpf_tcp_ops_set_rcvlowat().  Should this include a Fixes tag?

  Fixes: 5f549c1e49f3 ("bpf: tcp: Add kfunc to adjust sk->sk_rcvlowat.")

Also, the commit message ends with a trailing colon and references [2]
without a corresponding link, as if the trailers were truncated.

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Sashiko AI review · https://sashiko.dev/#/patchset/au27Y9m78_3EH2v1@gmail.com?part=1