From: Jakub Kicinski <jakub.kicinski@netronome.com>
To: Daniel Borkmann <daniel@iogearbox.net>
Cc: Thomas Richter <tmricht@linux.ibm.com>,
ast@kernel.org, netdev@vger.kernel.org,
linux-kernel@vger.kernel.org, heiko.carstens@de.ibm.com,
brueckner@linux.vnet.ibm.com, schwidefsky@de.ibm.com
Subject: Re: [PATCH] perf build: Build error in libbpf missing initialization
Date: Fri, 27 Jul 2018 12:56:32 -0700 [thread overview]
Message-ID: <20180727125632.33634af5@cakuba.netronome.com> (raw)
In-Reply-To: <c8409ab8-69df-17f4-9da3-610c60143d9a@iogearbox.net>
On Fri, 27 Jul 2018 21:31:01 +0200, Daniel Borkmann wrote:
> On 07/27/2018 07:59 PM, Jakub Kicinski wrote:
> > On Fri, 27 Jul 2018 10:21:26 +0200, Thomas Richter wrote:
> >> In linux-next tree compiling the perf tool with additional make flags
> >> "EXTRA_CFLAGS="-Wp,-D_FORTIFY_SOURCE=2 -O2"
> >> causes a compiler error. It is the warning
> >> 'variable may be used uninitialized'
> >> which is treated as error:
> >>
> >> I compile it using a FEDORA 28 installation, my gcc compiler version:
> >> gcc (GCC) 8.0.1 20180324 (Red Hat 8.0.1-0.20)
> >>
> >> The file that causes the error is tools/lib/bpf/libbpf.c
> >>
> >> Here is the error message:
> >>
> >> [root@p23lp27] # make V=1 EXTRA_CFLAGS="-Wp,-D_FORTIFY_SOURCE=2 -O2"
> >> [...]
> >> Makefile.config:849: No openjdk development package found, please
> >> install JDK package, e.g. openjdk-8-jdk, java-1.8.0-openjdk-devel
> >> Warning: Kernel ABI header at 'tools/include/uapi/linux/if_link.h'
> >> differs from latest version at 'include/uapi/linux/if_link.h'
> >> CC libbpf.o
> >> libbpf.c: In function ‘bpf_perf_event_read_simple’:
> >> libbpf.c:2342:6: error: ‘ret’ may be used uninitialized in this
> >> function [-Werror=maybe-uninitialized]
> >> int ret;
> >> ^
> >> cc1: all warnings being treated as errors
> >> mv: cannot stat './.libbpf.o.tmp': No such file or directory
> >> /home6/tmricht/linux-next/tools/build/Makefile.build:96: recipe for target 'libbpf.o' failed
> >>
> >> Fix this warning and add an addition check at the beginning
> >> of the while loop.
> >>
> >> Cc: Alexei Starovoitov <ast@kernel.org>
> >> Cc: Daniel Borkmann <daniel@iogearbox.net>
> >>
> >> Suggested-by: Jakub Kicinski <jakub.kicinski@netronome.com>
> >> Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
> >
> > Ah, you already sent this, LGTM, thanks Thomas!
> >
> >> tools/lib/bpf/libbpf.c | 2 ++
> >> 1 file changed, 2 insertions(+)
> >>
> >> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> >> index 73465caa33ba..66965ca96113 100644
> >> --- a/tools/lib/bpf/libbpf.c
> >> +++ b/tools/lib/bpf/libbpf.c
> >> @@ -2349,6 +2349,8 @@ bpf_perf_event_read_simple(void *mem, unsigned long size,
> >>
> >> begin = base + data_tail % size;
> >> end = base + data_head % size;
> >> + if (begin == end)
> >> + return LIBBPF_PERF_EVENT_ERROR;
> >>
> >> while (begin != end) {
> >> struct perf_event_header *ehdr;
>
> One question though, any objections to go for something like the below instead?
> I doubt we ever hit this in a 'normal' situation, and given we already test for
> the begin and end anyway, we could just avoid the extra test altogether. I could
> change it to the below if you're good as well (no need to resend anything):
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index d881d37..1aafdbe 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -2273,8 +2273,8 @@ bpf_perf_event_read_simple(void *mem, unsigned long size,
> volatile struct perf_event_mmap_page *header = mem;
> __u64 data_tail = header->data_tail;
> __u64 data_head = header->data_head;
> + int ret = LIBBPF_PERF_EVENT_ERROR;
> void *base, *begin, *end;
> - int ret;
>
> asm volatile("" ::: "memory"); /* in real code it should be smp_rmb() */
> if (data_head == data_tail)
No real objection, although as a matter of personal taste I'm not a big
fan of initializing err/ret variables unless the code is explicitly
structured to make use of it. Here it looks slightly more like
silencing a compiler warning, hence my preference to address the actual
cause of the warning rather than catch all. I guess one could argue
the other way, i.e. if the loop never run (and therefore ret was not
overwritten) there must be *some* error. I like verbose/explicit code I
guess..
Up to you :)
next prev parent reply other threads:[~2018-07-27 19:56 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-27 8:21 [PATCH] perf build: Build error in libbpf missing initialization Thomas Richter
2018-07-27 17:59 ` Jakub Kicinski
2018-07-27 19:31 ` Daniel Borkmann
2018-07-27 19:56 ` Jakub Kicinski [this message]
2018-07-28 19:30 ` Daniel Borkmann
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