perf tools: Fix int16_t truncation of max_cpu_num in set_max_cpu_num()

set_max_cpu_num() assigns the sysfs "possible" CPU count to
max_cpu_num.cpu which is int16_t (struct perf_cpu).  On systems
with >32767 possible CPUs the value silently truncates, potentially
wrapping negative.  This causes cpunode_map to be underallocated
and subsequent cpu__get_node() calls to read out of bounds.

The matching check for max_present_cpu_num was added by commit
c760174401f6 ("perf cpumap: Reduce cpu size from int to int16_t")
but max_cpu_num was missed.  Add the same INT16_MAX guard.

Fixes: c760174401f605cf ("perf cpumap: Reduce cpu size from int to int16_t")
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Cc: Ian Rogers <irogers@google.com>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

1 files changed, 17 insertions, 4 deletions

diff --git a/tools/perf/util/cpumap.c b/tools/perf/util/cpumap.c
index d3432622b2ad..21fa781b03cc 100644
--- a/tools/perf/util/cpumap.c
+++ b/tools/perf/util/cpumap.c
@@ -494,6 +494,16 @@ static void set_max_cpu_num(void)
 	if (ret)
 		goto out;
 
+	/*
+	 * struct perf_cpu.cpu is int16_t (libperf ABI) — clamp to avoid
+	 * truncation to negative.  See tools/lib/perf/TODO for the ABI
+	 * widening plan.
+	 */
+	if (max > INT16_MAX) {
+		pr_warning("WARNING: max possible cpus %d exceeds int16_t, clamping to %d\n",
+			   max, INT16_MAX);
+		max = INT16_MAX;
+	}
 	max_cpu_num.cpu = max;
 
 	/* get the highest present cpu number for a sparse allocation */
@@ -506,11 +516,12 @@ static void set_max_cpu_num(void)
 	ret = get_max_num(path, &max);
 
 	if (!ret && max > INT16_MAX) {
-		pr_err("Read out of bounds max cpus of %d\n", max);
-		ret = -1;
+		pr_warning("WARNING: max present cpus %d exceeds int16_t, clamping to %d\n",
+			   max, INT16_MAX);
+		max = INT16_MAX;
 	}
 	if (!ret)
-		max_present_cpu_num.cpu = (int16_t)max;
+		max_present_cpu_num.cpu = max;
 out:
 	if (ret)
 		pr_err("Failed to read max cpus, using default of %d\n", max_cpu_num.cpu);
@@ -647,7 +658,9 @@ int cpu__setup_cpunode_map(void)
 		while ((dent2 = readdir(dir2)) != NULL) {
 			if (dent2->d_type != DT_LNK || sscanf(dent2->d_name, "cpu%u", &cpu) < 1)
 				continue;
-			cpunode_map[cpu] = mem;
+			/* cpunode_map allocated for max_cpu_num entries */
+			if (cpu < (unsigned int)max_cpu_num.cpu)
+				cpunode_map[cpu] = mem;
 		}
 		closedir(dir2);
 	}