Messages in this thread | | | Date | Fri, 27 Jul 2018 10:57:57 -0700 | From | Jakub Kicinski <> | Subject | Re: [PATCH v2] perf build: Build error in libbpf with EXTRA_CFLAGS="-Wp,-D_FORTIFY_SOURCE=2 -O2" |
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On Fri, 27 Jul 2018 09:22:03 +0200, Thomas-Mich Richter wrote: > On 07/27/2018 04:16 AM, Daniel Borkmann wrote: > > On 07/26/2018 03:48 AM, Jakub Kicinski wrote: > >> On Wed, 25 Jul 2018 09:21:26 +0200, Thomas Richter wrote: > >>> commit a5b8bd47dcc57 ("bpf tools: Collect eBPF programs from their own sections") > >> > >> Hmm.. are you sure it's not 531b014e7a2f ("tools: bpf: make use of > >> reallocarray") that caused the issue? That commit made us switch from > >> XSI-compliant to GNU-specific strerror_r() implementation.. > >> > >> /me checks > >> > >> Yes it looks like 531b014e7a2f~ builds just fine. > >> > >> Daniel, did you try to apply v1 to the bpf tree? Perhaps there is a > >> confusion about the trees here, if this is caused by my recent change > >> it's a bpf-next material. strerror() works, but strerror_r() seems > >> nicer, so perhaps we could keep it if the patch worked in bpf-next? > > > > Yeah indeed, the issue is only in bpf-next. When I compile libbpf from > > bpf tree with the below flags then it's all good> > > Agree that we should rather use strerror_r() given this is a library. > > Are you sure to stick with strerror_r? I ask because it is the only > occurence of strerror_r in this file. All other error messages use strerror > as in: > pr_warning("failed to create map (name: '%s'): %s\n", > map->name, > strerror(errno)); > > > $ fgrep strerror tools/lib/bpf/libbpf.c > strerror(errno)); > issue I try to solve---> strerror_r(-err, errmsg, sizeof(errmsg)); > map->name, strerror(errno), errno); > strerror(errno)); > pr_warning("load bpf program failed: %s\n", strerror(errno)); > pr_warning("failed to statfs %s: %s\n", dir, strerror(errno)); > pr_warning("failed to pin program: %s\n", strerror(errno)); > pr_warning("failed to mkdir %s: %s\n", path, strerror(-err)); > pr_warning("failed to pin map: %s\n", strerror(errno)); > $ > > The next issue with strerror_r is to assign the return value to a variable. > Then we end up with variable set but not used: > libbpf.c: In function ‘bpf_object__elf_collect’: > libbpf.c:809:35: error: variable ‘cp’ set but not used [-Werror=unused-but-set-variable] > char errmsg[STRERR_BUFSIZE], *cp; > ^ > cc1: all warnings being treated as errors
The GNU-specific strerror_r() returns a pointer to a string containing the error message. This may be either a pointer to a string that the function stores in buf, or a pointer to some (immutable) static string (in which case buf is unused). If the function stores a string in buf, then at most buflen bytes are stored (the string may be truncated if buflen is too small and errnum is unknown). The string always includes a terminating null byte ('\0').
IOW you gotta use the return value.
> Here is the source: > if (err) { > char errmsg[STRERR_BUFSIZE], *cp; > > cp = strerror_r(-err, errmsg, sizeof(errmsg)); > pr_warning("failed to alloc program %s (%s): %s", > name, obj->path, errmsg); > } > > To fix this requires something like: > pr_warning("failed to alloc program %s (%s): %s", > name, obj->path, cp);
This looks correct.
> And after pr_warning() is done, the local array errmsg is deleted. > > A lot of overkill in my opinion, unless I miss something.
IMO using potentially mutli-thread unsafe functions in a library is not optimal, we should strive to convert the other instances of strerror() rather than move the other way.
> >>> causes a compiler error when building the perf tool in the linux-next tree. > >>> 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: > > [...] > >>> @@ -2334,7 +2331,7 @@ bpf_perf_event_read_simple(void *mem, unsigned long size, > >>> __u64 data_tail = header->data_tail; > >>> __u64 data_head = header->data_head; > >>> void *base, *begin, *end; > >>> - int ret; > >>> + int ret = 0; > >>> > >>> asm volatile("" ::: "memory"); /* in real code it should be smp_rmb() */ > >>> if (data_head == data_tail) > >> > >> This looks like a separate issue. The ret variable should really be > >> enum bpf_perf_event_ret, so could you please initialize it to one of the > >> values of this enum? > >> > >> The uninitilized condition can only happen if (data_head != data_tail) > >> but at the same time (data_head % size == data_tail % size) which > >> should never really happen... Perhaps initializing to > >> LIBBPF_PERF_EVENT_ERROR would make sense? > >> > >> Or better still adding: > >> > >> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c > >> index f732237610e5..fa5a25945f19 100644 > >> --- a/tools/lib/bpf/libbpf.c > >> +++ b/tools/lib/bpf/libbpf.c > >> @@ -2289,6 +2289,8 @@ bpf_perf_event_read_simple(void *mem, unsigned long size, > >> > >> begin = base + data_tail % size; > >> end = base + data_head % size; > >> + if (being == end) > >> + return LIBBPF_PERF_EVENT_ERROR; > > > > Sounds good to me. > > > > If you want I can send you a separate patch for this.
As far as I'm concerned - yes, please!
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