[PATCH] tools/perf/tests: Fix compilation error with strncpy in tests/tool_pmu

[Ian Rogers]

On Wed, Oct 16, 2024 at 11:23 AM Athira Rajeev
<atrajeev at linux.vnet.ibm.com> wrote:
>
>
>
> > On 16 Oct 2024, at 8:36 PM, Ian Rogers <irogers at google.com> wrote:
> >
> > On Wed, Oct 16, 2024 at 5:30 AM Athira Rajeev
> > <atrajeev at linux.vnet.ibm.com> wrote:
> >>
> >>
> >>
> >>> On 14 Oct 2024, at 10:56 PM, Namhyung Kim <namhyung at kernel.org> wrote:
> >>>
> >>> Hello Athira,
> >>>
> >>> On Sun, Oct 13, 2024 at 11:07:42PM +0530, Athira Rajeev wrote:
> >>>> perf fails to compile on systems with GCC version11
> >>>> as below:
> >>>>
> >>>> In file included from /usr/include/string.h:519,
> >>>>                from /home/athir/perf-tools-next/tools/include/linux/bitmap.h:5,
> >>>>                from /home/athir/perf-tools-next/tools/perf/util/pmu.h:5,
> >>>>                from /home/athir/perf-tools-next/tools/perf/util/evsel.h:14,
> >>>>                from /home/athir/perf-tools-next/tools/perf/util/evlist.h:14,
> >>>>                from tests/tool_pmu.c:3:
> >>>> In function ‘strncpy’,
> >>>>   inlined from ‘do_test’ at tests/tool_pmu.c:25:3:
> >>>> /usr/include/bits/string_fortified.h:95:10: error: ‘__builtin_strncpy’ specified bound 128 equals destination size [-Werror=stringop-truncation]
> >>>>  95 |   return __builtin___strncpy_chk (__dest, __src, __len,
> >>>>     |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >>>>  96 |                                   __glibc_objsize (__dest));
> >>>>     |                                   ~~~~~~~~~~~~~~~~~~~~~~~~~
> >>>>
> >>>> The compile error is from strncpy refernce in do_test:
> >>>> strncpy(str, tool_pmu__event_to_str(ev), sizeof(str));
> >>>>
> >>>> This behaviour is not observed with GCC version 8, but observed
> >>>> with GCC version 11 . This is message from gcc for detecting
> >>>> truncation while using strncpu. Use snprintf instead of strncpy
> >>>> here to be safe.
> >>>>
> >>>> Signed-off-by: Athira Rajeev <atrajeev at linux.vnet.ibm.com>
> >>>
> >>> I found this issue now and thanks for the quick fix.  I will push to
> >>> perf-tools-next soon.
> >>>
> >>> Thanks,
> >>> Namhyung
> >>>
> >>
> >> Sure
> >>
> >> Thanks Namhyung for pulling in the change
> >>
> >> Athira
> >
> > Thanks for the fixes. As this is test code I don't think performance,
> > style, etc. matter much. The GCC strncpy warnings are annoying imo,
> > I'm not sure it makes sense for them to be enabled. I see in the
> > kernel lots of "sizeof(foo)-1" as a workaround. strlcpy looks like a
> > better alternative but it gets a checkpatch warning as in the kernel
> > strscpy is preferred. Perhaps we should create a strscpy shim.
> >
>
> IIUC, strscpy is defined in kernel source and is currently not used in tools side.
>
> Ian,
> Your point here is that we can create an interface in tools side so that we can later safely use strscpy in cases like this ?

Yeah. Tbh the string code is a bit chaotic in the code base. There's
some stuff using char*s, some asprintf, there's strbuf then there all
the kernel ways of doing things. I lose track of why strscpy is
superior to strlcpy, but strlcpy will get you a checkpath warning.

The standard kernel solution to the GCC strncpy warning I've seen,
changing "strncpy(a, b, sizeof(a))" to "strncpy(a, b, sizeof(a) - 1)",
looks dodgy and possibly worse to me. The point of the warning I think
relates to strncpy not necessarily \0 terminating the string and you
need to reserve at least 1 character for this, while strlcpy
guarantees the \0 termination. Doing "sizeof(..) - 1" likely confuses
the checker making it give up, but now you have less space to copy a
potential \0 as well as not getting a \0.

Thanks,
Ian


> Thanks
> Athira
>
> > Thanks,
> > Ian
> >
> >>>> ---
> >>>> tools/perf/tests/tool_pmu.c | 2 +-
> >>>> 1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/tools/perf/tests/tool_pmu.c b/tools/perf/tests/tool_pmu.c
> >>>> index 94d0dd8fd3cb..297cc8c55579 100644
> >>>> --- a/tools/perf/tests/tool_pmu.c
> >>>> +++ b/tools/perf/tests/tool_pmu.c
> >>>> @@ -22,7 +22,7 @@ static int do_test(enum tool_pmu_event ev, bool with_pmu)
> >>>> if (with_pmu)
> >>>> snprintf(str, sizeof(str), "tool/%s/", tool_pmu__event_to_str(ev));
> >>>> else
> >>>> - strncpy(str, tool_pmu__event_to_str(ev), sizeof(str));
> >>>> + snprintf(str, sizeof(str), "%s", tool_pmu__event_to_str(ev));
> >>>>
> >>>> parse_events_error__init(&err);
> >>>> ret = parse_events(evlist, str, &err);
> >>>> --
> >>>> 2.43.5
>
>