iommu/vt-d: Replace snprintf with scnprintf in dmar_latency_snapshot()

snprintf() returns the number of bytes that would have been written, not
the number actually written. Using this for offset tracking can cause
buffer overruns if truncation occurs.

Replace snprintf() with scnprintf() to ensure the offset stays within
bounds.

Since scnprintf() never returns a negative value, and zero is not possible
in this context because 'bytes' starts at 0 and 'size - bytes' is
DEBUG_BUFFER_SIZE in the first call, which is large enough to hold the
string literals used, the return value is always positive. An integer
overflow is also completely out of reach here due to the small and fixed
buffer size. The error check in latency_show_one() is therefore
unnecessary. Remove it and make dmar_latency_snapshot() return void.

Signed-off-by: Seyediman Seyedarab <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Lu Baolu <[e-mail]>
Signed-off-by: Joerg Roedel <[e-mail]>

drivers/iommu/intel/debugfs.c

···
 static void latency_show_one(struct seq_file *m, struct intel_iommu *iommu,
 			     struct dmar_drhd_unit *drhd)
 {
-	int ret;
-
 	seq_printf(m, "IOMMU: %s Register Base Address: %llx\n",
 		   iommu->name, drhd->reg_base_addr);
 
-	ret = dmar_latency_snapshot(iommu, debug_buf, DEBUG_BUFFER_SIZE);
-	if (ret < 0)
-		seq_puts(m, "Failed to get latency snapshot");
-	else
-		seq_puts(m, debug_buf);
-	seq_puts(m, "\n");
+	dmar_latency_snapshot(iommu, debug_buf, DEBUG_BUFFER_SIZE);
+	seq_printf(m, "%s\n", debug_buf);
 }
 
 static int latency_show(struct seq_file *m, void *v)

drivers/iommu/intel/perf.c

···
 	"     svm_prq"
 };
 
-int dmar_latency_snapshot(struct intel_iommu *iommu, char *str, size_t size)
+void dmar_latency_snapshot(struct intel_iommu *iommu, char *str, size_t size)
 {
 	struct latency_statistic *lstat = iommu->perf_statistic;
 	unsigned long flags;
···
 	memset(str, 0, size);
 
 	for (i = 0; i < COUNTS_NUM; i++)
-		bytes += snprintf(str + bytes, size - bytes,
+		bytes += scnprintf(str + bytes, size - bytes,
 				  "%s", latency_counter_names[i]);
 
 	spin_lock_irqsave(&latency_lock, flags);
···
 		if (!dmar_latency_enabled(iommu, i))
 			continue;
 
-		bytes += snprintf(str + bytes, size - bytes,
+		bytes += scnprintf(str + bytes, size - bytes,
 				  "\n%s", latency_type_names[i]);
 
 		for (j = 0; j < COUNTS_NUM; j++) {
···
 				break;
 			}
 
-			bytes += snprintf(str + bytes, size - bytes,
+			bytes += scnprintf(str + bytes, size - bytes,
 					  "%12lld", val);
 		}
 	}
 	spin_unlock_irqrestore(&latency_lock, flags);
-
-	return bytes;
 }

drivers/iommu/intel/perf.h

···
 bool dmar_latency_enabled(struct intel_iommu *iommu, enum latency_type type);
 void dmar_latency_update(struct intel_iommu *iommu, enum latency_type type,
 			 u64 latency);
-int dmar_latency_snapshot(struct intel_iommu *iommu, char *str, size_t size);
+void dmar_latency_snapshot(struct intel_iommu *iommu, char *str, size_t size);
 #else
 static inline int
 dmar_latency_enable(struct intel_iommu *iommu, enum latency_type type)
···
 {
 }
 
-static inline int
+static inline void
 dmar_latency_snapshot(struct intel_iommu *iommu, char *str, size_t size)
 {
-	return 0;
 }
 #endif /* CONFIG_DMAR_PERF */