seq_file: always update file->f_pos in seq_lseek()

Gu Zheng · Al Viro · commit 05e16745c0c4 · 2013-10-25T10:46:40.000-04:00
This issue was first pointed out by Jiaxing Wang several months ago, but no further comments: https://lkml.org/lkml/2013/6/29/41 As we know pread() does not change f_pos, so after pread(), file->f_pos and m->read_pos become different. And seq_lseek() does not update file->f_pos if offset equals to m->read_pos, so after pread() and seq_lseek()(lseek to m->read_pos), then a subsequent read may read from a wrong position, the following program produces the problem: char str1[32] = { 0 }; char str2[32] = { 0 }; int poffset = 10; int count = 20; /*open any seq file*/ int fd = open("/proc/modules", O_RDONLY); pread(fd, str1, count, poffset); printf("pread:%s\n", str1); /*seek to where m->read_pos is*/ lseek(fd, poffset+count, SEEK_SET); /*supposed to read from poffset+count, but this read from position 0*/ read(fd, str2, count); printf("read:%s\n", str2); out put: pread: ck_netbios_ns 12665 read: nf_conntrack_netbios /proc/modules: nf_conntrack_netbios_ns 12665 0 - Live 0xffffffffa038b000 nf_conntrack_broadcast 12589 1 nf_conntrack_netbios_ns, Live 0xffffffffa0386000 So we always update file->f_pos to offset in seq_lseek() to fix this issue. Signed-off-by: Jiaxing Wang <hello.wjx@gmail.com> Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/seq_file.c b/fs/seq_file.c
@@ -328,6 +328,8 @@ loff_t seq_lseek(struct file *file, loff_t offset, int whence)
 				m->read_pos = offset;
 				retval = file->f_pos = offset;
 			}
+		} else {
+			file->f_pos = offset;
 		}
 	}
 	file->f_version = m->version;

Original file line number	Diff line number	Diff line change
`@@ -328,6 +328,8 @@ loff_t seq_lseek(struct file *file, loff_t offset, int whence)`
`328`	`328`	`m->read_pos = offset;`
`329`	`329`	`retval = file->f_pos = offset;`
`330`	`330`	`}`
	`331`	`+ } else {`
	`332`	`+ file->f_pos = offset;`
`331`	`333`	`}`
`332`	`334`	`}`
`333`	`335`	`file->f_version = m->version;`