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Commit 7904ac84 authored by Earl Chew's avatar Earl Chew Committed by Linus Torvalds
Browse files

seq_file: fix mishandling of consecutive pread() invocations. 

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.

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



Signed-off-by: default avatarEarl Chew <echew@ixiacom.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent dc716e96

fs/seq_file.c

+15 −13
Original line number Diff line number Diff line
@@ -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);