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SubjectRe: [PATCH 1/1] Fix seq_file mishandling of consecutive pread() invocations.
[ Added Maintainers; Added reference to bugzilla.kernel.org in commit log ]

Also reported in:

https://bugzilla.kernel.org/show_bug.cgi?id=11856

The following program illustrates the problem:

char buf[8192];

int fd = open("/proc/self/maps", O_RDONLY);

n = pread(fd, buf, sizeof(buf), 0);
printf("%d\n", n);

/* lseek(fd, 0, SEEK_CUR); */ /* Uncomment to work around */
n = pread(fd, buf, sizeof(buf), 0);
printf("%d\n", n);

The second printf() prints zero, but uncommenting the lseek()
corrects its behaviour.

To fix, make seq_read() mirror seq_lseek() when processing changes in *ppos.
Restore m->version first, then if required traverse and update read_pos
on success.

Signed-off-by: Earl Chew <echew@ixiacom.com>
---
fs/seq_file.c | 28 +++++++++++++++-------------
1 files changed, 15 insertions(+), 13 deletions(-)
diff --git a/fs/seq_file.c b/fs/seq_file.c
index dba43c3..7a45306 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -140,9 +140,21 @@ ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)

mutex_lock(&m->lock);

+ /*
+ * seq_file->op->..m_start/m_stop/m_next may do special actions
+ * or optimisations based on the file->f_version, so we want to
+ * pass the file->f_version to those methods.
+ *
+ * seq_file->version is just copy of f_version, and seq_file
+ * methods can treat it simply as file version.
+ * It is copied in first and copied out after all operations.
+ * It is convenient to have it as part of structure to avoid the
+ * need of passing another argument to all the seq_file methods.
+ */
+ m->version = file->f_version;
+
/* Don't assume *ppos is where we left it */
if (unlikely(*ppos != m->read_pos)) {
- m->read_pos = *ppos;
while ((err = traverse(m, *ppos)) == -EAGAIN)
;
if (err) {
@@ -152,21 +164,11 @@ ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
m->index = 0;
m->count = 0;
goto Done;
+ } else {
+ m->read_pos = *ppos;
}
}

- /*
- * seq_file->op->..m_start/m_stop/m_next may do special actions
- * or optimisations based on the file->f_version, so we want to
- * pass the file->f_version to those methods.
- *
- * seq_file->version is just copy of f_version, and seq_file
- * methods can treat it simply as file version.
- * It is copied in first and copied out after all operations.
- * It is convenient to have it as part of structure to avoid the
- * need of passing another argument to all the seq_file methods.
- */
- m->version = file->f_version;
/* grab buffer if we didn't have one */
if (!m->buf) {
m->buf = kmalloc(m->size = PAGE_SIZE, GFP_KERNEL);
--
1.7.0.4



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