linux-kernel - Re: [PATCH v2] kbuild: fix SIGPIPE error message for AR=gcc-ar and AR=llvm-ar

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Date:   Thu, 17 Nov 2022 05:37:31 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     linux-kbuild@...r.kernel.org, Jiri Slaby <jirislaby@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nicolas Schier <nicolas@...sle.eu>, Tom Rix <trix@...hat.com>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v2] kbuild: fix SIGPIPE error message for AR=gcc-ar and AR=llvm-ar

On Thu, Nov 17, 2022 at 4:01 AM Kees Cook <keescook@...omium.org> wrote:
>
> On Fri, Oct 28, 2022 at 01:28:39AM +0900, Masahiro Yamada wrote:
> > Jiri Slaby reported that building the kernel with AR=gcc-ar shows:
> >   /usr/bin/ar terminated with signal 13 [Broken pipe]
> >
> > Nathan Chancellor reported the latest AR=llvm-ar shows
> >   error: write on a pipe with no reader
> >
> > The latter occurs since LLVM commit 51b557adc131 ("Add an error message
> > to the default SIGPIPE handler").
> >
> > The resulting vmlinux is correct, but it is better to silence it.
> >
> > 'head -n1' exits after reading the first line, so the pipe is closed.
> >
> > Use 'sed -n 1p' to eat the stream till the end.
>
> I think this is wrong because it needlessly consumes CPU time. SIGPIPE
> is _needed_ to stop a process after we found what we needed, but it's up
> to the caller (the shell here) to determine what to do about it.
>
> Similarly, that LLVM commit is wrong -- tools should _not_ catch their
> own SIGPIPEs. They should be caught by their callers.
>
> For example, see:
>
> $ seq 10000 | head -n1
> 1
>
> ^^^ no warnings from the shell (caller of "seq")
> And you can see it _is_ being killed by SIGPIPE:
>
> $ strace seq 1000 | head -n1
> ...
> write(1, "1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n11\n12\n13\n14"..., 8192) = 8192
> 1
> write(1, "\n1861\n1862\n1863\n1864\n1865\n1866\n1"..., 4096) = -1 EPIPE (Broken pipe)
> --- SIGPIPE {si_signo=SIGPIPE, si_code=SI_USER, si_pid=3503448, si_uid=1000} ---
> +++ killed by SIGPIPE +++
>
> If we use "sed -n 1p" seq will continue to run, consuming needless time
> and CPU resources.
>
> So, I strongly think this is the wrong solution. SIGPIPE should be
> ignored for ar, and LLVM should _not_ catch its own SIGPIPE.
>
> -Kees


I thought of this - it is just wasting CPU time,
but I did not come up with a better idea on the kbuild side.

I do not want to use 2>/dev/null because it may hide
non-SIGPIPE (i.e. real) errors.


I think you guys will be keen on fixing llvm.
I hope gcc as well?



-- 
Best Regards
Masahiro Yamada