linux-kernel - [PATCH] perf/x86/amd/uncore: Avoid a false positive warning about snprintf truncation in amd_uncore_umc_ctx

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Message-ID: <20241104141406.3fd11254@endymion.delvare>
Date: Mon, 4 Nov 2024 14:14:06 +0100
From: Jean Delvare <jdelvare@...e.de>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Sandipan Das <sandipan.das@....com>, Peter Zijlstra
 <peterz@...radead.org>
Subject: [PATCH] perf/x86/amd/uncore: Avoid a false positive warning about
 snprintf truncation in amd_uncore_umc_ctx_init

Fix the following warning:
  CC [M]  arch/x86/events/amd/uncore.o
arch/x86/events/amd/uncore.c: In function ‘amd_uncore_umc_ctx_init’:
arch/x86/events/amd/uncore.c:951:52: warning: ‘%d’ directive output may be truncated writing between 1 and 10 bytes into a region of size 8 [-Wformat-truncation=]
    snprintf(pmu->name, sizeof(pmu->name), "amd_umc_%d", index);
                                                    ^~
arch/x86/events/amd/uncore.c:951:43: note: directive argument in the range [0, 2147483647]
    snprintf(pmu->name, sizeof(pmu->name), "amd_umc_%d", index);
                                           ^~~~~~~~~~~~
arch/x86/events/amd/uncore.c:951:4: note: ‘snprintf’ output between 10 and 19 bytes into a destination of size 16
    snprintf(pmu->name, sizeof(pmu->name), "amd_umc_%d", index);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

As far as I can see, there can't be more than UNCORE_GROUP_MAX (256)
groups and each group can't have more than 255 PMU, so the number
printed by this %d can't exceed 65279, that's only 5 digits and would
fit into the buffer. So it's a false positive warning. But we can
make the compiler happy by declaring index as a 16-bit number.

Signed-off-by: Jean Delvare <jdelvare@...e.de>
---
An alternative fix would be to extend UNCORE_NAME_LEN to 20, the
downside being an increased memory consumption. Depends whether we
expect UNCORE_GROUP_MAX to ever be increased, or groups to ever
support more than 255 PMU.

 arch/x86/events/amd/uncore.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- linux-6.12-rc6.orig/arch/x86/events/amd/uncore.c
+++ linux-6.12-rc6/arch/x86/events/amd/uncore.c
@@ -916,7 +916,8 @@ int amd_uncore_umc_ctx_init(struct amd_u
 	u8 group_num_pmcs[UNCORE_GROUP_MAX] = { 0 };
 	union amd_uncore_info info;
 	struct amd_uncore_pmu *pmu;
-	int index = 0, gid, i;
+	int gid, i;
+	u16 index = 0;
 
 	if (pmu_version < 2)
 		return 0;

-- 
Jean Delvare
SUSE L3 Support