Re: [PATCH] perf tools: Fix arm64 build error with gcc-11

[Leo Yan]

Hi Jianlin,

On Tue, Feb 09, 2021 at 07:33:57PM +0800, Jianlin Lv wrote:
> gcc version: 11.0.0 20210208 (experimental) (GCC)
> 
> Following build error on arm64:
> 
> .......
> In function ‘printf’,
>     inlined from ‘regs_dump__printf’ at util/session.c:1141:3,
>     inlined from ‘regs__printf’ at util/session.c:1169:2:
> /usr/include/aarch64-linux-gnu/bits/stdio2.h:107:10: \
>   error: ‘%-5s’ directive argument is null [-Werror=format-overflow=]
> 
> 107 |   return __printf_chk (__USE_FORTIFY_LEVEL - 1, __fmt, \
>                 __va_arg_pack ());
> 
> ......
> In function ‘fprintf’,
>   inlined from ‘perf_sample__fprintf_regs.isra’ at \
>     builtin-script.c:622:14:
> /usr/include/aarch64-linux-gnu/bits/stdio2.h:100:10: \
> 	error: ‘%5s’ directive argument is null [-Werror=format-overflow=]
>   100 |   return __fprintf_chk (__stream, __USE_FORTIFY_LEVEL - 1, __fmt,
>   101 |                         __va_arg_pack ());
> 
> cc1: all warnings being treated as errors
> .......
> 
> This patch fixes Wformat-overflow warnings by replacing the return
> value NULL of perf_reg_name with "unknown".
> 
> Signed-off-by: Jianlin Lv <Jianlin.Lv@xxxxxxx>
> ---
>  tools/perf/arch/arm64/include/perf_regs.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/perf/arch/arm64/include/perf_regs.h b/tools/perf/arch/arm64/include/perf_regs.h
> index baaa5e64a3fb..901419f907c0 100644
> --- a/tools/perf/arch/arm64/include/perf_regs.h
> +++ b/tools/perf/arch/arm64/include/perf_regs.h
> @@ -85,10 +85,10 @@ static inline const char *perf_reg_name(int id)
>  	case PERF_REG_ARM64_PC:
>  		return "pc";
>  	default:
> -		return NULL;
> +		return "unknown";
>  	}
>  
> -	return NULL;
> +	return "unknown";

This issue is a common issue crossing all archs.  So it's better to
change the code in the places where calls perf_reg_name(), e.g. in
util/session.c:

--- a/tools/perf/util/session.c
+++ b/tools/perf/util/session.c
@@ -1135,12 +1135,14 @@ static void branch_stack__printf(struct perf_sample *sample, bool callstack)
 static void regs_dump__printf(u64 mask, u64 *regs)
 {
        unsigned rid, i = 0;
+       char *reg_name;
 
        for_each_set_bit(rid, (unsigned long *) &mask, sizeof(mask) * 8) {
                u64 val = regs[i++];
 
+               reg_name = perf_reg_name(rid);
                printf(".... %-5s 0x%016" PRIx64 "\n",
-                      perf_reg_name(rid), val);
+                      reg_name ?: "Unknown", val);
        }
 }

And another potential issue is the format specifier "%-5s", it prints
out maximum to 5 chars, but actually string "Unknown" has 7 chars.
Actually the format specifier breaks other archs register names, e.g.
[1][2], seems to me, it's better to change as "%-8s", you might need
to use a dedicated patch for format specifier changes.

Thanks,
Leo


[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/perf/arch/powerpc/include/perf_regs.h#n57
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/perf/arch/csky/include/perf_regs.h#n83