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Subject[PATCH] filp->f_pos not correctly updated in proc_task_readdir
Date
filp->f_pos only get updated at the end of the function. Thus d_off of those
dirents who are in the middle will be 0, and this will cause a problem in
glibc's readdir implementation, specifically endless loop. Because when overflow
occurs, f_pos will be set to next dirent to read, however it will be 0, unless
the next one is the last one. So it will start over again and again.

There is a sample program in man 2 gendents. This is the output of the program
running on a multithread program's task dir before this patch is applied:

$ ./a.out /proc/3807/task
--------------- nread=128 ---------------
i-node# file type d_reclen d_off d_name
506442 directory 16 1 .
506441 directory 16 0 ..
506443 directory 16 0 3807
506444 directory 16 0 3809
506445 directory 16 0 3812
506446 directory 16 0 3861
506447 directory 16 0 3862
506448 directory 16 8 3863
This is the output after this patch is applied

$ ./a.out /proc/3807/task
--------------- nread=128 ---------------
i-node# file type d_reclen d_off d_name
506442 directory 16 1 .
506441 directory 16 2 ..
506443 directory 16 3 3807
506444 directory 16 4 3809
506445 directory 16 5 3812
506446 directory 16 6 3861
506447 directory 16 7 3862
506448 directory 16 8 3863
Signed-off-by: Zhang Le <r0bertz@gentoo.org>
---
fs/proc/base.c | 16 +++++++---------
1 files changed, 7 insertions(+), 9 deletions(-)
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 0c9de19..cc6ea23 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -3066,7 +3066,6 @@ static int proc_task_readdir(struct file * filp, void * dirent, filldir_t filldi
int retval = -ENOENT;
ino_t ino;
int tid;
- unsigned long pos = filp->f_pos; /* avoiding "long long" filp->f_pos */
struct pid_namespace *ns;

task = get_proc_task(inode);
@@ -3083,18 +3082,18 @@ static int proc_task_readdir(struct file * filp, void * dirent, filldir_t filldi
goto out_no_task;
retval = 0;

- switch (pos) {
+ switch (filp->f_pos) {
case 0:
ino = inode->i_ino;
- if (filldir(dirent, ".", 1, pos, ino, DT_DIR) < 0)
+ if (filldir(dirent, ".", 1, filp->f_pos, ino, DT_DIR) < 0)
goto out;
- pos++;
+ filp->f_pos++;
/* fall through */
case 1:
ino = parent_ino(dentry);
- if (filldir(dirent, "..", 2, pos, ino, DT_DIR) < 0)
+ if (filldir(dirent, "..", 2, filp->f_pos, ino, DT_DIR) < 0)
goto out;
- pos++;
+ filp->f_pos++;
/* fall through */
}

@@ -3104,9 +3103,9 @@ static int proc_task_readdir(struct file * filp, void * dirent, filldir_t filldi
ns = filp->f_dentry->d_sb->s_fs_info;
tid = (int)filp->f_version;
filp->f_version = 0;
- for (task = first_tid(leader, tid, pos - 2, ns);
+ for (task = first_tid(leader, tid, filp->f_pos - 2, ns);
task;
- task = next_tid(task), pos++) {
+ task = next_tid(task), filp->f_pos++) {
tid = task_pid_nr_ns(task, ns);
if (proc_task_fill_cache(filp, dirent, filldir, task, tid) < 0) {
/* returning this tgid failed, save it as the first
@@ -3117,7 +3116,6 @@ static int proc_task_readdir(struct file * filp, void * dirent, filldir_t filldi
}
}
out:
- filp->f_pos = pos;
put_task_struct(leader);
out_no_task:
return retval;
--
1.6.2


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