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

[Jianlin Lv]

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";
 }
 
 #endif /* ARCH_PERF_REGS_H */
-- 
2.25.1