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pidref: record pidfd inode number in PidRef struct

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Besides internal comparisons, the inode number of pidfds
might be interesting directly to users, too. In the future
this field should also be exposed, so that it can serve as
a unique identifier of a process (but only for display,
as there's no method to map this back to a pid or pidfd).
This commit is contained in:
Mike Yuan 2024-05-23 22:19:05 +08:00
parent 1b6239632d
commit 15930d5d9f
No known key found for this signature in database
GPG Key ID: 417471C0A40F58B3
3 changed files with 48 additions and 21 deletions
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48
src/basic/pidref.c
View File

@ -32,9 +32,38 @@ static int pidfd_inode_ids_supported(void) {
return (cached = fd_is_fs_type(fd, PID_FS_MAGIC));
}
bool pidref_equal(const PidRef *a, const PidRef *b) {
int pidref_acquire_pidfd_id(PidRef *pidref) {
int r;
assert(pidref);
if (!pidref_is_set(pidref))
return -ESRCH;
if (pidref->fd < 0)
return -ENOMEDIUM;
if (pidref->fd_id > 0)
return 0;
r = pidfd_inode_ids_supported();
if (r < 0)
return r;
if (r == 0)
return -EOPNOTSUPP;
struct stat st;
if (fstat(pidref->fd, &st) < 0)
return log_debug_errno(errno, "Failed to get inode number of pidfd for pid " PID_FMT ": %m",
pidref->pid);
pidref->fd_id = (uint64_t) st.st_ino;
return 0;
}
bool pidref_equal(PidRef *a, PidRef *b) {
if (pidref_is_set(a)) {
if (!pidref_is_set(b))
return false;
@ -42,20 +71,13 @@ bool pidref_equal(const PidRef *a, const PidRef *b) {
if (a->pid != b->pid)
return false;
if (a->fd < 0 || b->fd < 0)
/* Try to compare pidfds using their inode numbers. This way we can ensure that we don't
* spuriously consider two PidRefs equal if the pid has been reused once. Note that we
* ignore all errors here, not only EOPNOTSUPP, as fstat() might fail due to many reasons. */
if (pidref_acquire_pidfd_id(a) < 0 || pidref_acquire_pidfd_id(b) < 0)
return true;
/* pidfds live in their own pidfs and each process comes with a unique inode number since
* kernel 6.9. */
if (pidfd_inode_ids_supported() <= 0)
return true;
r = fd_inode_same(a->fd, b->fd);
if (r < 0)
log_debug_errno(r, "Failed to check whether pidfds for pid " PID_FMT " are equal, assuming yes: %m",
a->pid);
return r != 0;
return a->fd_id == b->fd_id;
}
return !pidref_is_set(b);

9
src/basic/pidref.h
View File

@ -5,8 +5,10 @@
/* An embeddable structure carrying a reference to a process. Supposed to be used when tracking processes continuously. */
typedef struct PidRef {
pid_t pid; /* always valid */
int fd; /* only valid if pidfd are available in the kernel, and we manage to get an fd */
pid_t pid; /* always valid */
int fd; /* only valid if pidfd are available in the kernel, and we manage to get an fd */
uint64_t fd_id; /* the inode number of pidfd. only useful in kernel 6.9+ where pidfds live in
their own pidfs and each process comes with a unique inode number */
} PidRef;
#define PIDREF_NULL (const PidRef) { .fd = -EBADF }
@ -19,7 +21,8 @@ static inline bool pidref_is_set(const PidRef *pidref) {
return pidref && pidref->pid > 0;
}
bool pidref_equal(const PidRef *a, const PidRef *b);
int pidref_acquire_pidfd_id(PidRef *pidref);
bool pidref_equal(PidRef *a, PidRef *b);
/* This turns a pid_t into a PidRef structure, and acquires a pidfd for it, if possible. (As opposed to
* PIDREF_MAKE_FROM_PID() above, which does not acquire a pidfd.) */

12
src/test/test-pidref.c
View File

@ -7,6 +7,8 @@
#include "stdio-util.h"
#include "tests.h"
#define PIDREF_NULL_NONCONST (PidRef) { .fd = -EBADF }
TEST(pidref_is_set) {
assert_se(!pidref_is_set(NULL));
assert_se(!pidref_is_set(&PIDREF_NULL));
@ -15,14 +17,14 @@ TEST(pidref_is_set) {
TEST(pidref_equal) {
assert_se(pidref_equal(NULL, NULL));
assert_se(pidref_equal(NULL, &PIDREF_NULL));
assert_se(pidref_equal(&PIDREF_NULL, NULL));
assert_se(pidref_equal(&PIDREF_NULL, &PIDREF_NULL));
assert_se(pidref_equal(NULL, &PIDREF_NULL_NONCONST));
assert_se(pidref_equal(&PIDREF_NULL_NONCONST, NULL));
assert_se(pidref_equal(&PIDREF_NULL_NONCONST, &PIDREF_NULL_NONCONST));
assert_se(!pidref_equal(NULL, &PIDREF_MAKE_FROM_PID(1)));
assert_se(!pidref_equal(&PIDREF_MAKE_FROM_PID(1), NULL));
assert_se(!pidref_equal(&PIDREF_NULL, &PIDREF_MAKE_FROM_PID(1)));
assert_se(!pidref_equal(&PIDREF_MAKE_FROM_PID(1), &PIDREF_NULL));
assert_se(!pidref_equal(&PIDREF_NULL_NONCONST, &PIDREF_MAKE_FROM_PID(1)));
assert_se(!pidref_equal(&PIDREF_MAKE_FROM_PID(1), &PIDREF_NULL_NONCONST));
assert_se(pidref_equal(&PIDREF_MAKE_FROM_PID(1), &PIDREF_MAKE_FROM_PID(1)));
assert_se(!pidref_equal(&PIDREF_MAKE_FROM_PID(1), &PIDREF_MAKE_FROM_PID(2)));
}