[Libguestfs] [PATCH] umask: Use /proc/<PID>/status to read umask in Linux >= 4.7.
Since Linux 4.7, the process umask is available in /proc/<pid>/status.
See:
https://github.com/torvalds/linux/commit/3e42979e65dace1f9268dd5440e5ab09...
Use this value if available, else fall back to the existing codepath
for Linux <= 4.6 and other Unix.
---
src/umask.c | 86 +++++++++++++++++++++++++++++++++++++++++++++++++++++++------
1 file changed, 78 insertions(+), 8 deletions(-)
diff --git a/src/umask.c b/src/umask.c
index b8748e8..3f32337 100644
--- a/src/umask.c
+++ b/src/umask.c
@@ -18,12 +18,22 @@
/**
* Return current umask in a thread-safe way.
+ *
+ * glibc documents, but does not actually implement, a "getumask(3)"
+ * call.
+ *
+ * We use C<Umask> from F</proc/I<PID>/status> for Linux E<ge>
4.7.
+ * For older Linux and other Unix, this file implements an expensive
+ * but thread-safe way to get the current process's umask.
+ *
+ * Thanks to: Josh Stone, Jiri Jaburek, Eric Blake.
*/
#include <config.h>
#include <stdio.h>
#include <stdlib.h>
+#include <stdbool.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
@@ -35,21 +45,81 @@
#include "guestfs.h"
#include "guestfs-internal.h"
+static int get_umask_from_proc (guestfs_h *g);
+static int get_umask_from_fork (guestfs_h *g);
+
/**
- * glibc documents, but does not actually implement, a L<getumask(3)>
- * call.
- *
- * This function implements an expensive, but thread-safe way to get
- * the current process's umask.
- *
* Returns the current process's umask. On failure, returns C<-1> and
* sets the error in the guestfs handle.
- *
- * Thanks to: Josh Stone, Jiri Jaburek, Eric Blake.
*/
int
guestfs_int_getumask (guestfs_h *g)
{
+ int mask;
+
+ mask = get_umask_from_proc (g);
+ if (mask == -1)
+ return -1;
+ if (mask >= 0)
+ return mask;
+
+ return get_umask_from_fork (g);
+}
+
+/**
+ * For Linux E<ge> 4.7 get the umask from F</proc/I<PID>/status>.
+ *
+ * On failure this returns C<-1>. However if we could not open the
+ * F</proc> file or find the C<Umask> entry in it, return C<-2> which
+ * causes the fallback path to run.
+ */
+static int
+get_umask_from_proc (guestfs_h *g)
+{
+ FILE *fp;
+ CLEANUP_FREE char *path = NULL, *line = NULL;
+ size_t allocsize;
+ ssize_t len;
+ unsigned int mask;
+ bool found = false;
+
+ path = safe_asprintf (g, "/proc/%d/status", getpid ());
+ fp = fopen (path, "r");
+ if (fp == NULL) {
+ if (errno == ENOENT || errno == ENOTDIR)
+ return -2; /* fallback */
+ perrorf (g, "open: %s", path);
+ return -1;
+ }
+
+ while ((len = getline (&line, &allocsize, fp)) != -1) {
+ if (len > 0 && line[len-1] == '\n')
+ line[--len] = '\0';
+
+ /* Looking for: "Umask: 0022" */
+ if (sscanf (line, "Umask: %o", &mask) == 1) {
+ found = true;
+ break;
+ }
+ }
+
+ if (fclose (fp) == -1) {
+ perrorf (g, "close: %s", path);
+ return -1;
+ }
+
+ if (!found)
+ return -2; /* fallback */
+
+ return (int) mask;
+}
+
+/**
+ * Fallback method of getting the umask using fork.
+ */
+static int
+get_umask_from_fork (guestfs_h *g)
+{
pid_t pid;
int fd[2], r;
int mask;
--
2.7.4