Re: [PATCH bpf-next v2] libbpf: fix build error introduced by legacy kprobe feature

On Tue, Sep 14, 2021 at 2:39 PM Rafael David Tinoco
[off-list ref] wrote:

quoted

-     char cmd[192], probename[128], probefunc[128];
+     char cmd[288] = "\0", probename[128] = "\0", probefunc[128] = "\0";
      const char *file = "/sys/kernel/debug/tracing/kprobe_events";

I had gcc-10 with:

libbpf.c: In function ‘poke_kprobe_events’:
libbpf.c:9012:37: error: ‘%s’ directive output may be truncated writing up to 127 bytes into a region of size between 62 and 189 [-Werror=format-truncation=]
 9012 |   snprintf(cmd, sizeof(cmd), "%c:%s %s",
      |                                     ^~
In file included from /usr/include/stdio.h:866,
                 from libbpf.c:17:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:71:10: note: ‘__builtin___snprintf_chk’ output between 4 and 258 bytes into a destination of size 192
   71 |   return __builtin___snprintf_chk (__s, __n, __USE_FORTIFY_LEVEL - 1,
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   72 |        __glibc_objsize (__s), __fmt,
      |        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   73 |        __va_arg_pack ());
      |        ~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

locally AND it fixed the issue for me but Alexei reported:

https://github.com/kernel-patches/bpf/runs/3603448190

with a truncation of max 258 bytes. I raised cmd size to 288.

Right, if offset != 0 GCC is able to deduce that sizeof(probename) +
sizeof(probefunc) add up to 256 (plus few more characters in format
string) and that will be truncated in cmd. It completely ignores such
possibility when offset == 0 and we use name as is. Either way, I
bumped the cmd size to 256 and force-pushed. It should build fine now.
Sorry for delay.

Let's see if that fixes the issue.