Messages in this thread | | | Date | Fri, 27 Jul 2018 12:56:32 -0700 | From | Jakub Kicinski <> | Subject | Re: [PATCH] perf build: Build error in libbpf missing initialization |
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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 :)
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