Messages in this thread | | | Subject | Re: [PATCH] perf build: Build error in libbpf missing initialization | From | Daniel Borkmann <> | Date | Sat, 28 Jul 2018 21:30:34 +0200 |
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On 07/27/2018 09:56 PM, Jakub Kicinski wrote: > 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 :)
Ok, I pushed this variant out to the bpf tree since it also is affected there. Thanks a lot everyone!
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