[PATCH v2] selftests/pid_namespace: compute pid_max test limits dynamically

[PATCH v2] selftests/pid_namespace: compute pid_max test limits dynamically

[Bjoern Doebel]

The pid_max kselftest hardcodes pid_max values of 400 and 500, but the
kernel enforces a minimum of PIDS_PER_CPU_MIN * num_possible_cpus().
On machines with many possible CPUs (e.g. nr_cpu_ids=128 yields a
minimum of 1024), writing 400 or 500 to /proc/sys/kernel/pid_max
returns EINVAL and all three tests fail.

Compute these limits the same way as the kernel does and set outer_limit
and inner_limit dynamically based on the result. Original test semantics
are preserved (outer < inner, nested namespace capped by parent).

Signed-off-by: Bjoern Doebel <doebel@amazon.com>
Assisted-by: Kiro:claude-opus-4.6
---
v2:
- use global outer_limit/inner_limit instead of complicated config
  struct
- make use of FIXTURE/TEST_F macros
- reduce buffer size in write_int_to_fd() to 12

 .../testing/selftests/pid_namespace/pid_max.c | 156 ++++++++++++++----
 1 file changed, 124 insertions(+), 32 deletions(-)

diff --git a/tools/testing/selftests/pid_namespace/pid_max.c b/tools/testing/selftests/pid_namespace/pid_max.c
index c9519e7385b6..5d686a09aa15 100644
--- a/tools/testing/selftests/pid_namespace/pid_max.c
+++ b/tools/testing/selftests/pid_namespace/pid_max.c
@@ -12,10 +12,74 @@
 #include <syscall.h>
 #include <sys/mount.h>
 #include <sys/wait.h>
+#include <unistd.h>
 
 #include "kselftest_harness.h"
 #include "../pidfd/pidfd.h"
 
+/*
+ * The kernel computes the minimum allowed pid_max as:
+ *   max(RESERVED_PIDS + 1, PIDS_PER_CPU_MIN * num_possible_cpus())
+ * Mirror that here so the test values are always valid.
+ *
+ * Note: glibc's get_nprocs_conf() returns the number of *configured*
+ * (present) CPUs, not *possible* CPUs.  The kernel uses
+ * num_possible_cpus() which corresponds to /sys/devices/system/cpu/possible.
+ * These can differ significantly (e.g. 16 configured vs 128 possible).
+ */
+#define RESERVED_PIDS		300
+#define PIDS_PER_CPU_MIN	8
+
+/* Count CPUs from a range list like "0-31" or "0-15,32-47". */
+static int num_possible_cpus(void)
+{
+	FILE *f;
+	int count = 0;
+	int lo, hi;
+
+	f = fopen("/sys/devices/system/cpu/possible", "r");
+	if (!f)
+		return 0;
+
+	while (fscanf(f, "%d", &lo) == 1) {
+		if (fscanf(f, "-%d", &hi) == 1)
+			count += hi - lo + 1;
+		else
+			count++;
+		/* skip comma separator */
+		fscanf(f, ",");
+	}
+
+	fclose(f);
+	return count;
+}
+
+static int pid_min(void)
+{
+	int cpu_min = PIDS_PER_CPU_MIN * num_possible_cpus();
+
+	return cpu_min > (RESERVED_PIDS + 1) ? cpu_min : (RESERVED_PIDS + 1);
+}
+
+/*
+ * Outer and inner pid_max limits used by the tests.  The outer limit is
+ * the more restrictive ancestor; the inner limit is set higher in a
+ * nested namespace but must still be capped by the outer limit.
+ * Both are derived from the kernel's minimum so they are always writable.
+ *
+ * Global so that clone callbacks can access them without parameter plumbing.
+ */
+static int outer_limit;
+static int inner_limit;
+
+static int write_int_to_fd(int fd, int val)
+{
+	char buf[12];
+	int len = snprintf(buf, sizeof(buf), "%d", val);
+
+	return write(fd, buf, len);
+}
+
 #define __STACK_SIZE (8 * 1024 * 1024)
 static pid_t do_clone(int (*fn)(void *), void *arg, int flags)
 {
@@ -60,18 +124,18 @@ static int pid_max_cb(void *data)
 		return -1;
 	}
 
-	ret = write(fd, "500", sizeof("500") - 1);
+	ret = write_int_to_fd(fd, inner_limit);
 	if (ret < 0) {
 		fprintf(stderr, "%m - Failed to write pid_max\n");
 		return -1;
 	}
 
-	for (int i = 0; i < 501; i++) {
+	for (int i = 0; i < inner_limit + 1; i++) {
 		pid = fork();
 		if (pid == 0)
 			exit(EXIT_SUCCESS);
 		wait_for_pid(pid);
-		if (pid > 500) {
+		if (pid > inner_limit) {
 			fprintf(stderr, "Managed to create pid number beyond limit\n");
 			return -1;
 		}
@@ -106,7 +170,7 @@ static int pid_max_nested_inner(void *data)
 		return fret;
 	}
 
-	ret = write(fd, "500", sizeof("500") - 1);
+	ret = write_int_to_fd(fd, inner_limit);
 	close(fd);
 	if (ret < 0) {
 		fprintf(stderr, "%m - Failed to write pid_max\n");
@@ -133,8 +197,8 @@ static int pid_max_nested_inner(void *data)
 		return fret;
 	}
 
-	/* Now make sure that we wrap pids at 400. */
-	for (i = 0; i < 510; i++) {
+	/* Now make sure that we wrap pids at outer_limit. */
+	for (i = 0; i < inner_limit + 10; i++) {
 		pid_t pid;
 
 		pid = fork();
@@ -145,7 +209,7 @@ static int pid_max_nested_inner(void *data)
 			exit(EXIT_SUCCESS);
 
 		wait_for_pid(pid);
-		if (pid >= 500) {
+		if (pid >= inner_limit) {
 			fprintf(stderr, "Managed to create process with pid %d beyond configured limit\n", pid);
 			return fret;
 		}
@@ -156,15 +220,19 @@ static int pid_max_nested_inner(void *data)
 
 static int pid_max_nested_outer(void *data)
 {
-	int fret = -1, nr_procs = 400;
-	pid_t pids[1000];
-	int fd, i, ret;
+	int fret = -1, nr_procs = 0;
+	pid_t *pids;
+	int fd, ret;
 	pid_t pid;
 
+	pids = malloc(outer_limit * sizeof(pid_t));
+	if (!pids)
+		return -1;
+
 	ret = mount("", "/", NULL, MS_PRIVATE | MS_REC, 0);
 	if (ret) {
 		fprintf(stderr, "%m - Failed to make rootfs private mount\n");
-		return fret;
+		goto out;
 	}
 
 	umount2("/proc", MNT_DETACH);
@@ -172,27 +240,28 @@ static int pid_max_nested_outer(void *data)
 	ret = mount("proc", "/proc", "proc", 0, NULL);
 	if (ret) {
 		fprintf(stderr, "%m - Failed to mount proc\n");
-		return fret;
+		goto out;
 	}
 
 	fd = open("/proc/sys/kernel/pid_max", O_RDWR | O_CLOEXEC | O_NOCTTY);
 	if (fd < 0) {
 		fprintf(stderr, "%m - Failed to open pid_max\n");
-		return fret;
+		goto out;
 	}
 
-	ret = write(fd, "400", sizeof("400") - 1);
+	ret = write_int_to_fd(fd, outer_limit);
 	close(fd);
 	if (ret < 0) {
 		fprintf(stderr, "%m - Failed to write pid_max\n");
-		return fret;
+		goto out;
 	}
 
 	/*
-	 * Create 397 processes. This leaves room for do_clone() (398) and
-	 * one more 399. So creating another process needs to fail.
+	 * Create (outer_limit - 4) processes. This leaves room for
+	 * do_clone() and one more. So creating another process needs
+	 * to fail.
 	 */
-	for (nr_procs = 0; nr_procs < 396; nr_procs++) {
+	for (nr_procs = 0; nr_procs < outer_limit - 4; nr_procs++) {
 		pid = fork();
 		if (pid < 0)
 			goto reap;
@@ -220,20 +289,26 @@ static int pid_max_nested_outer(void *data)
 	for (int i = 0; i < nr_procs; i++)
 		wait_for_pid(pids[i]);
 
+out:
+	free(pids);
 	return fret;
 }
 
 static int pid_max_nested_limit_inner(void *data)
 {
-	int fret = -1, nr_procs = 400;
+	int fret = -1, nr_procs = 0;
 	int fd, ret;
 	pid_t pid;
-	pid_t pids[1000];
+	pid_t *pids;
+
+	pids = malloc(inner_limit * sizeof(pid_t));
+	if (!pids)
+		return -1;
 
 	ret = mount("", "/", NULL, MS_PRIVATE | MS_REC, 0);
 	if (ret) {
 		fprintf(stderr, "%m - Failed to make rootfs private mount\n");
-		return fret;
+		goto out;
 	}
 
 	umount2("/proc", MNT_DETACH);
@@ -241,23 +316,23 @@ static int pid_max_nested_limit_inner(void *data)
 	ret = mount("proc", "/proc", "proc", 0, NULL);
 	if (ret) {
 		fprintf(stderr, "%m - Failed to mount proc\n");
-		return fret;
+		goto out;
 	}
 
 	fd = open("/proc/sys/kernel/pid_max", O_RDWR | O_CLOEXEC | O_NOCTTY);
 	if (fd < 0) {
 		fprintf(stderr, "%m - Failed to open pid_max\n");
-		return fret;
+		goto out;
 	}
 
-	ret = write(fd, "500", sizeof("500") - 1);
+	ret = write_int_to_fd(fd, inner_limit);
 	close(fd);
 	if (ret < 0) {
 		fprintf(stderr, "%m - Failed to write pid_max\n");
-		return fret;
+		goto out;
 	}
 
-	for (nr_procs = 0; nr_procs < 500; nr_procs++) {
+	for (nr_procs = 0; nr_procs < inner_limit; nr_procs++) {
 		pid = fork();
 		if (pid < 0)
 			break;
@@ -268,7 +343,7 @@ static int pid_max_nested_limit_inner(void *data)
 		pids[nr_procs] = pid;
 	}
 
-	if (nr_procs >= 400) {
+	if (nr_procs >= outer_limit) {
 		fprintf(stderr, "Managed to create processes beyond the configured outer limit\n");
 		goto reap;
 	}
@@ -279,6 +354,8 @@ static int pid_max_nested_limit_inner(void *data)
 	for (int i = 0; i < nr_procs; i++)
 		wait_for_pid(pids[i]);
 
+out:
+	free(pids);
 	return fret;
 }
 
@@ -307,7 +384,7 @@ static int pid_max_nested_limit_outer(void *data)
 		return -1;
 	}
 
-	ret = write(fd, "400", sizeof("400") - 1);
+	ret = write_int_to_fd(fd, outer_limit);
 	close(fd);
 	if (ret < 0) {
 		fprintf(stderr, "%m - Failed to write pid_max\n");
@@ -328,17 +405,32 @@ static int pid_max_nested_limit_outer(void *data)
 	return 0;
 }
 
-TEST(pid_max_simple)
+FIXTURE(pid_max) {
+	int dummy;
+};
+
+FIXTURE_SETUP(pid_max)
 {
-	pid_t pid;
+	int min = pid_min();
 
+	outer_limit = min + 100;
+	inner_limit = min + 200;
+}
+
+FIXTURE_TEARDOWN(pid_max)
+{
+}
+
+TEST_F(pid_max, simple)
+{
+	pid_t pid;
 
 	pid = do_clone(pid_max_cb, NULL, CLONE_NEWPID | CLONE_NEWNS);
 	ASSERT_GT(pid, 0);
 	ASSERT_EQ(0, wait_for_pid(pid));
 }
 
-TEST(pid_max_nested_limit)
+TEST_F(pid_max, nested_limit)
 {
 	pid_t pid;
 
@@ -347,7 +439,7 @@ TEST(pid_max_nested_limit)
 	ASSERT_EQ(0, wait_for_pid(pid));
 }
 
-TEST(pid_max_nested)
+TEST_F(pid_max, nested)
 {
 	pid_t pid;
 
-- 
2.50.1




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