Messages in this thread Patch in this message | | | Date | Sun, 22 Jan 2012 11:01:21 -0800 | From | Earl Chew <> | Subject | Re: [PATCH 1/1] Fix seq_file mishandling of consecutive pread() invocations. |
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[ 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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