* [PATCH v2] selftests/pid_namespace: compute pid_max test limits dynamically
[not found] <20260421194344.2981537-1-doebel@amazon.com>
@ 2026-04-22 20:11 ` Bjoern Doebel
0 siblings, 0 replies; only message in thread
From: Bjoern Doebel @ 2026-04-22 20:11 UTC (permalink / raw)
To: linux-kselftest
Cc: brauner, ptikhomirov, shuah, linux-kernel, stable, 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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