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

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Date:   Tue, 6 Dec 2022 13:24:41 +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 7:07 AM Kees Cook <keescook@...omium.org> wrote:
>
> On Thu, Nov 17, 2022 at 05:37:31AM +0900, Masahiro Yamada wrote:
> > 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.
>
> Yes, I've opened an upstream LLVM bug for this:
> https://github.com/llvm/llvm-project/issues/59037
>
> --
> Kees Cook



BTW, Python does something similar by default.
(noisy back-trace for SIGPIPE)





masahiro@zoe:/tmp$ cat test.py
#!/usr/bin/python3
for i in range(4000):
    print(i)

masahiro@zoe:/tmp$ ./test.py  |  head -n1
0
Traceback (most recent call last):
  File "/tmp/./test.py", line 3, in <module>
    print(i)
BrokenPipeError: [Errno 32] Broken pipe






This page
https://www.geeksforgeeks.org/broken-pipe-error-in-python/

suggests some workarounds.





Python scripts potentially have this issue.






$ ./scripts/diffconfig  .config.old  .config  | head -n1
-104_QUAD_8 m
Traceback (most recent call last):
  File "/home/masahiro/ref/linux/./scripts/diffconfig", line 132, in <module>
    main()
  File "/home/masahiro/ref/linux/./scripts/diffconfig", line 111, in main
    print_config("-", config, a[config], None)
  File "/home/masahiro/ref/linux/./scripts/diffconfig", line 62, in print_config
    print("-%s %s" % (config, value))
BrokenPipeError: [Errno 32] Broken pipe







What would you suggest for python scripts?






-- 
Best Regards
Masahiro Yamada