util/proc: add helper to resolve a pid file descriptor

A pid FD can be resolved by reading from proc's fdinfo

Signed-off-by: Luca Boccassi <[email protected]>
(rework based on the new proc helpers)
Signed-off-by: David Rheinsberg <[email protected]>

src/util/proc.c

@@ -8,6 +8,15 @@
 #include <unistd.h>
 #include "util/error.h"
 #include "util/proc.h"
+#include "util/string.h"
+
+/*
+ * A file in /proc can be at most 4M minus one. If required, we start with a 4K
+ * read, then try a 4M one if that fails. In the vast majority of cases, 4K
+ * will be enough.
+ */
+#define PROC_SIZE_MIN (4U*1024U)
+#define PROC_SIZE_MAX (4U*1024U*1024U - 1U)
 
 /**
  * proc_field() - Extract individual field from proc-text-file
@@ -176,3 +185,29 @@ int proc_get_seclabel(pid_t pid, char **labelp, size_t *n_labelp) {
  *labelp = label;
  return 0;
 }
+
+int proc_resolve_pidfd(int pidfd, pid_t *pidp) {
+ _c_cleanup_(c_freep) char *data = NULL, *field = NULL;
+ _c_cleanup_(c_closep) int fd = -1;
+ char path[64];
+ int r;
+
+ sprintf(path, "/proc/self/fdinfo/%d", pidfd);
+ fd = open(path, O_RDONLY | O_CLOEXEC);
+ if (fd < 0)
+ return error_origin(-errno);
+
+ r = proc_read(fd, &data, NULL);
+ if (r)
+ return error_fold(r);
+
+ r = proc_field(data, "Pid", &field);
+ if (r)
+ return error_fold(r);
+
+ r = util_strtoint(pidp, field);
+ if (r)
+ return error_fold(r);
+
+ return 0;
+}

src/util/proc.h

@@ -20,3 +20,4 @@ int proc_field(const char *data, const char *key, char **valuep);
 int proc_read(int fd, char **datap, size_t *n_datap);
 
 int proc_get_seclabel(pid_t pid, char **labelp, size_t *n_labelp);
+int proc_resolve_pidfd(int pidfd, pid_t *pidp);

src/util/test-proc.c

@@ -84,8 +84,27 @@ static void test_read(void) {
  c_free(data);
 }
 
+static void test_resolve_pidfd(void) {
+ _c_cleanup_(c_closep) int pidfd = -1;
+ pid_t pid;
+ int r;
+
+ pidfd = syscall_pidfd_open(getpid(), 0);
+ if (pidfd < 0) {
+ c_assert(errno == ENOSYS);
+ return;
+ }
+
+ c_assert(pidfd >= 0);
+
+ r = proc_resolve_pidfd(pidfd, &pid);
+ c_assert(!r);
+ c_assert(pid == getpid());
+}
+
 int main(int argc, char **argv) {
  test_field();
  test_read();
+ test_resolve_pidfd();
  return 0;
 }