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selftests/pid_namespace: compute pid_max test limits dynamically

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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>
Link: https://patch.msgid.link/20260422201151.3830506-1-doebel@amazon.com
Reviewed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Assisted-by: Kiro:claude-opus-4.6
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Bjoern Doebel 2026-04-22 20:11:51 +00:00 committed by Christian Brauner
parent 254f49634e
commit d324c5416a
No known key found for this signature in database
GPG Key ID: 91C61BC06578DCA2
1 changed files with 124 additions and 32 deletions
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156
tools/testing/selftests/pid_namespace/pid_max.c
View File

@ -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)
{
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;