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

[Kees Cook <keescook@chromium.org>]

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

> 
> Fixes: 321648455061 ("kbuild: use obj-y instead extra-y for objects placed at the head")
> Link: https://github.com/ClangBuiltLinux/linux/issues/1651
> Reported-by: Jiri Slaby <jirislaby@kernel.org>
> Reported-by: Nathan Chancellor <nathan@kernel.org>
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> Tested-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
> 
> Changes in v2:
>   - Update commit description to mention llvm-ar
> 
>  Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/Makefile b/Makefile
> index e90bb2b38607..e9e7eff906a5 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1218,7 +1218,7 @@ quiet_cmd_ar_vmlinux.a = AR      $@
>        cmd_ar_vmlinux.a = \
>  	rm -f $@; \
>  	$(AR) cDPrST $@ $(KBUILD_VMLINUX_OBJS); \
> -	$(AR) mPiT $$($(AR) t $@ | head -n1) $@ $$($(AR) t $@ | grep -F -f $(srctree)/scripts/head-object-list.txt)
> +	$(AR) mPiT $$($(AR) t $@ | sed -n 1p) $@ $$($(AR) t $@ | grep -F -f $(srctree)/scripts/head-object-list.txt)
>  
>  targets += vmlinux.a
>  vmlinux.a: $(KBUILD_VMLINUX_OBJS) scripts/head-object-list.txt autoksyms_recursive FORCE
> -- 
> 2.34.1
> 

-- 
Kees Cook