From: Eric Dumazet <eduma...@google.com>
__fget() makes sure a file refcount is not zero before
taking a reference. It should also fetch again file pointer
in order to respect dup2() atomicity requirements.
It should either read a NULL pointer or a file on which
a refcount can be taken.
Dmitry had following test failing sometimes :
#include <pthread.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
int fd;
void *Thread(void *x) {
char buf;
int n = read(fd, &buf, 1);
if (n != 1)
exit(printf("read failed: n=%d errno=%d\n", n, errno));
return 0;
}
int main()
{
fd = open("/dev/urandom", O_RDONLY);
int fd2 = open("/dev/urandom", O_RDONLY);
if (fd == -1 || fd2 == -1)
exit(printf("open failed\n"));
pthread_t th;
pthread_create(&th, 0, Thread, 0);
if (dup2(fd2, fd) == -1)
exit(printf("dup2 failed\n"));
pthread_join(th, 0);
if (close(fd) == -1)
exit(printf("close failed\n"));
if (close(fd2) == -1)
exit(printf("close failed\n"));
printf("DONE\n");
return 0;
}
Signed-off-by: Eric Dumazet <eduma...@google.com>
Reported-by: Dmitry Vyukov <dvyu...@google.com>
---
fs/file.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)
diff --git a/fs/file.c b/fs/file.c
index 93c5f89c248b..492bd74c4433 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -635,11 +635,17 @@ static struct file *__fget(unsigned int fd, fmode_t mask)
struct file *file;
rcu_read_lock();
+loop:
file = fcheck_files(files, fd);
if (file) {
- /* File object ref couldn't be taken */
- if ((file->f_mode & mask) || !get_file_rcu(file))
+ /* File object ref couldn't be taken.
+ * dup2() atomicity guarantee is the reason
+ * we loop to catch the new file (or NULL pointer)
+ */
+ if (file->f_mode & mask)
file = NULL;
+ else if (!get_file_rcu(file))
+ goto loop;
}
rcu_read_unlock();
--
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