seq_file: Fix overflow condition in seq_commit

[Arun KS]

seq_path()/seq_commit() is treating a d_path() failure as an overflow
condition, but it isn't.

seq_read handles overflow by reallocating more buffer. And this
continues in a loop utill we increase the size of seq buf beyond
KMALLOC_MAX_SIZE and hence a WARN_ON.

[  363.192565] ------------[ cut here ]------------
[  363.199462] WARNING: at mm/slab_common.c:377 kmalloc_slab+0x34/0x9c()
[  363.208557] Modules linked in:
[  363.212219] CPU: 1 PID: 1742 Comm: Binder_2 Tainted: G        W3.10.0+ #17
[  363.222930] [<c00151c4>] (unwind_backtrace+0x0/0x11c)
from[<c0011a24>] (show_stack+0x10/0x14)
[  363.235229] [<c0011a24>] (show_stack+0x10/0x14) from
[<c0059fb0>](warn_slowpath_common+0x4c/0x68)
[  363.247253] [<c0059fb0>] (warn_slowpath_common+0x4c/0x68)
from[<c0059fe4>] (warn_slowpath_null+0x18/0x1c)
[  363.259307] [<c0059fe4>] (warn_slowpath_null+0x18/0x1c)
from[<c00fa400>] (kmalloc_slab+0x34/0x9c)
[  363.270812] [<c00fa400>] (kmalloc_slab+0x34/0x9c) from
[<c010ec20>](__kmalloc+0x14/0x1fc)
[  363.281433] [<c010ec20>] (__kmalloc+0x14/0x1fc) from
[<c012ef70>](seq_read+0x24c/0x438)
[  363.291992] [<c012ef70>] (seq_read+0x24c/0x438) from
[<c011389c>](vfs_read+0xac/0x124)
[  363.302398] [<c011389c>] (vfs_read+0xac/0x124) from
[<c0113a38>](SyS_read+0x3c/0x60)
[  363.312591] [<c0113a38>] (SyS_read+0x3c/0x60) from
[<c000e180>](ret_fast_syscall+0x0/0x48)
[  363.323303] ---[ end trace 46c6467e2db7bcd4 ]---

Pass -ENOBUFS to seq_commit to signal an overflow condition and a
negative value for all other errors.

Signed-off-by: Arun KS <arun.ks@xxxxxxxxxxxx>
---
 fs/seq_file.c            |  6 +++---
 include/linux/seq_file.h | 12 +++++++-----
 2 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/fs/seq_file.c b/fs/seq_file.c
index 3135c25..6a33f9c 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -463,7 +463,7 @@ int seq_path(struct seq_file *m, const struct path
*path, const char *esc)
 {
  char *buf;
  size_t size = seq_get_buf(m, &buf);
- int res = -1;
+ int res = -ENOBUFS;

  if (size) {
  char *p = d_path(path, buf, size);
@@ -487,7 +487,7 @@ int seq_path_root(struct seq_file *m, const struct
path *path,
 {
  char *buf;
  size_t size = seq_get_buf(m, &buf);
- int res = -ENAMETOOLONG;
+ int res = -ENOBUFS;

  if (size) {
  char *p;
@@ -516,7 +516,7 @@ int seq_dentry(struct seq_file *m, struct dentry
*dentry, const char *esc)
 {
  char *buf;
  size_t size = seq_get_buf(m, &buf);
- int res = -1;
+ int res = -ENOBUFS;

  if (size) {
  char *p = dentry_path(dentry, buf, size);
diff --git a/include/linux/seq_file.h b/include/linux/seq_file.h
index 4e32edc..43f51a0 100644
--- a/include/linux/seq_file.h
+++ b/include/linux/seq_file.h
@@ -7,6 +7,7 @@
 #include <linux/mutex.h>
 #include <linux/cpumask.h>
 #include <linux/nodemask.h>
+#include <linux/errno.h>

 struct seq_operations;
 struct file;
@@ -66,16 +67,17 @@ static inline size_t seq_get_buf(struct seq_file
*m, char **bufp)
  * @num: the number of bytes to commit
  *
  * Commit @num bytes of data written to a buffer previously acquired
- * by seq_buf_get.  To signal an error condition, or that the data
- * didn't fit in the available space, pass a negative @num value.
+ * by seq_buf_get.  To signal an overflow condition(data didn't fit
+ * in the available space), pass -ENOBUFS and for other errors pass a
+ * negative @num value.
  */
 static inline void seq_commit(struct seq_file *m, int num)
 {
- if (num < 0) {
- m->count = m->size;
- } else {
+ if (num >= 0) {
  BUG_ON(m->count + num > m->size);
  m->count += num;
+ } else if (num == -ENOBUFS)
+ m->count = m->size;
  }
 }

-- 
1.8.2

--047d7b343f1aa22de604e45a4906
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