Libguestfs - May 2016 - [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/3e42979e65dace1f9268dd5440e5ab096b8dee59

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

On Saturday 21 May 2016 13:14:40 Richard W.M. Jones
wrote:
> Since Linux 4.7, the process umask is available in
/proc/<pid>/status.
> See:
>
https://github.com/torvalds/linux/commit/3e42979e65dace1f9268dd5440e5ab096b8dee59
> 
> Use this value if available, else fall back to the existing codepath
> for Linux <= 4.6 and other Unix.
> ---
Sounds good -- just wondering whether the result of the inspection of
the "status" file should be cached somehow, so following get_umask
calls
after the first one will use only the working method.

A couple of notes follow.
>  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;
There's CLEANUP_FCLOSE, although it discards the error on fclose()
(which could be ignored, anyway?).
> +  CLEANUP_FREE char *path = NULL, *line = NULL;
> +  size_t allocsize;
This needs to be reset to 0, as documented for getline.
> +  ssize_t len;
> +  unsigned int mask;
> +  bool found = false;
> +
> +  path = safe_asprintf (g, "/proc/%d/status", getpid ());
This does not need a new buffer on heap, a short stack buffer like
  char path[sizeof ("/proc/%d/status") + sizeof (pid_t) * 3];
should be enough.
> +  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;
> 
-- 
Pino Toscano