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[198.0.35.241]) by smtp.gmail.com with ESMTPSA id z19-20020a630a53000000b00476db6fe510sm2287032pgk.75.2022.11.16.11.00.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 16 Nov 2022 11:00:59 -0800 (PST) Date: Wed, 16 Nov 2022 11:00:58 -0800 From: Kees Cook To: Masahiro Yamada Cc: linux-kbuild@vger.kernel.org, Jiri Slaby , Nathan Chancellor , Nick Desaulniers , Michal Marek , Nicolas Schier , Tom Rix , linux-kernel@vger.kernel.org, llvm@lists.linux.dev Subject: Re: [PATCH v2] kbuild: fix SIGPIPE error message for AR=gcc-ar and AR=llvm-ar Message-ID: <202211161056.1B9611A@keescook> References: <20221027162839.410720-1-masahiroy@kernel.org> Precedence: bulk X-Mailing-List: llvm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20221027162839.410720-1-masahiroy@kernel.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 > Reported-by: Nathan Chancellor > Signed-off-by: Masahiro Yamada > Tested-by: Nick Desaulniers > --- > > 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