On 4/21/26 21:43, Bjoern Doebel wrote:
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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 <[email protected]>
> ---
> .../testing/selftests/pid_namespace/pid_max.c | 153 ++++++++++++++----
> 1 file changed, 119 insertions(+), 34 deletions(-)
>
> diff --git a/tools/testing/selftests/pid_namespace/pid_max.c
> b/tools/testing/selftests/pid_namespace/pid_max.c
> index c9519e7385b6..8bd600f55421 100644
> --- a/tools/testing/selftests/pid_namespace/pid_max.c
> +++ b/tools/testing/selftests/pid_namespace/pid_max.c
> @@ -12,10 +12,76 @@
> #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);
> +}
> +
> +struct pid_max_cfg {
> + int outer;
> + int inner;
> +};
> +
> +/*
> + * 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.
> + */
> +#define PID_MAX_CFG_INIT { .outer = pid_min() + 100, .inner = pid_min() +
> 200 }
> +
> +static int write_int_to_fd(int fd, int val)
> +{
> + char buf[24];
Why 24? Since val is int and longest int is INT_MIN, which has 11 characters,
so 12 should be enough.
> + 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)
> {
> @@ -37,6 +103,7 @@ static pid_t do_clone(int (*fn)(void *), void *arg, int
> flags)
>
> static int pid_max_cb(void *data)
> {
> + struct pid_max_cfg *cfg = data;
> int fd, ret;
> pid_t pid;
>
> @@ -60,18 +127,18 @@ static int pid_max_cb(void *data)
> return -1;
> }
>
> - ret = write(fd, "500", sizeof("500") - 1);
> + ret = write_int_to_fd(fd, cfg->inner);
> 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 < cfg->inner + 1; i++) {
> pid = fork();
> if (pid == 0)
> exit(EXIT_SUCCESS);
> wait_for_pid(pid);
> - if (pid > 500) {
> + if (pid > cfg->inner) {
> fprintf(stderr, "Managed to create pid number beyond
> limit\n");
> return -1;
> }
> @@ -82,6 +149,7 @@ static int pid_max_cb(void *data)
>
> static int pid_max_nested_inner(void *data)
> {
> + struct pid_max_cfg *cfg = data;
> int fret = -1;
> pid_t pids[2];
> int fd, i, ret;
> @@ -106,7 +174,7 @@ static int pid_max_nested_inner(void *data)
> return fret;
> }
>
> - ret = write(fd, "500", sizeof("500") - 1);
> + ret = write_int_to_fd(fd, cfg->inner);
> close(fd);
> if (ret < 0) {
> fprintf(stderr, "%m - Failed to write pid_max\n");
> @@ -133,8 +201,8 @@ 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 < cfg->inner + 10; i++) {
> pid_t pid;
>
> pid = fork();
> @@ -145,7 +213,7 @@ static int pid_max_nested_inner(void *data)
> exit(EXIT_SUCCESS);
>
> wait_for_pid(pid);
> - if (pid >= 500) {
> + if (pid >= cfg->inner) {
> fprintf(stderr, "Managed to create process with pid %d
> beyond configured limit\n", pid);
> return fret;
> }
> @@ -156,15 +224,20 @@ 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;
> + struct pid_max_cfg *cfg = data;
> + int fret = -1, nr_procs = 0;
> + pid_t *pids;
> + int fd, ret;
> pid_t pid;
>
> + pids = malloc(cfg->outer * 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 +245,27 @@ 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, cfg->outer);
> 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 - 4) processes. This leaves room for do_clone()
> + * and one more process. So creating another process needs to fail.
> */
> - for (nr_procs = 0; nr_procs < 396; nr_procs++) {
> + for (nr_procs = 0; nr_procs < cfg->outer - 4; nr_procs++) {
> pid = fork();
> if (pid < 0)
> goto reap;
> @@ -203,7 +276,7 @@ static int pid_max_nested_outer(void *data)
> pids[nr_procs] = pid;
> }
>
> - pid = do_clone(pid_max_nested_inner, NULL, CLONE_NEWPID | CLONE_NEWNS);
> + pid = do_clone(pid_max_nested_inner, cfg, CLONE_NEWPID | CLONE_NEWNS);
> if (pid < 0) {
> fprintf(stderr, "%m - Failed to clone nested pidns\n");
> goto reap;
> @@ -220,20 +293,27 @@ 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;
> + struct pid_max_cfg *cfg = data;
> + int fret = -1, nr_procs = 0;
> int fd, ret;
> pid_t pid;
> - pid_t pids[1000];
> + pid_t *pids;
> +
> + pids = malloc(cfg->inner * 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 +321,23 @@ 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, cfg->inner);
> 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 < cfg->inner; nr_procs++) {
> pid = fork();
> if (pid < 0)
> break;
> @@ -268,7 +348,7 @@ static int pid_max_nested_limit_inner(void *data)
> pids[nr_procs] = pid;
> }
>
> - if (nr_procs >= 400) {
> + if (nr_procs >= cfg->outer) {
> fprintf(stderr, "Managed to create processes beyond the
> configured outer limit\n");
> goto reap;
> }
> @@ -279,11 +359,14 @@ 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;
> }
>
> static int pid_max_nested_limit_outer(void *data)
> {
> + struct pid_max_cfg *cfg = data;
> int fd, ret;
> pid_t pid;
>
> @@ -307,14 +390,14 @@ static int pid_max_nested_limit_outer(void *data)
> return -1;
> }
>
> - ret = write(fd, "400", sizeof("400") - 1);
> + ret = write_int_to_fd(fd, cfg->outer);
> close(fd);
> if (ret < 0) {
> fprintf(stderr, "%m - Failed to write pid_max\n");
> return -1;
> }
>
> - pid = do_clone(pid_max_nested_limit_inner, NULL, CLONE_NEWPID |
> CLONE_NEWNS);
> + pid = do_clone(pid_max_nested_limit_inner, cfg, CLONE_NEWPID |
> CLONE_NEWNS);
> if (pid < 0) {
> fprintf(stderr, "%m - Failed to clone nested pidns\n");
> return -1;
> @@ -330,28 +413,30 @@ static int pid_max_nested_limit_outer(void *data)
>
> TEST(pid_max_simple)
> {
> + struct pid_max_cfg cfg = PID_MAX_CFG_INIT;
Maybe we can simplify things by using global variable instead of pushing
argument everywhere?
There is also FIXTURE_SETUP/TEST_F which can probably be combined with global
variable
too. Plus you can try to avoid calling pid_min() multiple times.
> pid_t pid;
>
> -
> - pid = do_clone(pid_max_cb, NULL, CLONE_NEWPID | CLONE_NEWNS);
> + pid = do_clone(pid_max_cb, &cfg, CLONE_NEWPID | CLONE_NEWNS);
> ASSERT_GT(pid, 0);
> ASSERT_EQ(0, wait_for_pid(pid));
> }
>
> TEST(pid_max_nested_limit)
> {
> + struct pid_max_cfg cfg = PID_MAX_CFG_INIT;
> pid_t pid;
>
> - pid = do_clone(pid_max_nested_limit_outer, NULL, CLONE_NEWPID |
> CLONE_NEWNS);
> + pid = do_clone(pid_max_nested_limit_outer, &cfg, CLONE_NEWPID |
> CLONE_NEWNS);
> ASSERT_GT(pid, 0);
> ASSERT_EQ(0, wait_for_pid(pid));
> }
>
> TEST(pid_max_nested)
> {
> + struct pid_max_cfg cfg = PID_MAX_CFG_INIT;
> pid_t pid;
>
> - pid = do_clone(pid_max_nested_outer, NULL, CLONE_NEWPID | CLONE_NEWNS);
> + pid = do_clone(pid_max_nested_outer, &cfg, CLONE_NEWPID | CLONE_NEWNS);
> ASSERT_GT(pid, 0);
> ASSERT_EQ(0, wait_for_pid(pid));
> }
--
Best regards, Pavel Tikhomirov
Senior Software Developer, Virtuozzo.
