diff options
authorNicolai Stange <nicstange@gmail.com>2016-03-25 14:22:14 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2016-03-25 16:37:42 -0700
commite7080a439a6f507abbc860847c33efc39b5c1c6d (patch)
parent9dcadd381b1d199074937019d612346c061de415 (diff)
mm/filemap: generic_file_read_iter(): check for zero reads unconditionally
If - generic_file_read_iter() gets called with a zero read length, - the read offset is at a page boundary, - IOCB_DIRECT is not set - and the page in question hasn't made it into the page cache yet, then do_generic_file_read() will trigger a readahead with a req_size hint of zero. Since roundup_pow_of_two(0) is undefined, UBSAN reports UBSAN: Undefined behaviour in include/linux/log2.h:63:13 shift exponent 64 is too large for 64-bit type 'long unsigned int' CPU: 3 PID: 1017 Comm: sa1 Tainted: G L 4.5.0-next-20160318+ #14 [...] Call Trace: [...] [<ffffffff813ef61a>] ondemand_readahead+0x3aa/0x3d0 [<ffffffff813ef61a>] ? ondemand_readahead+0x3aa/0x3d0 [<ffffffff813c73bd>] ? find_get_entry+0x2d/0x210 [<ffffffff813ef9c3>] page_cache_sync_readahead+0x63/0xa0 [<ffffffff813cc04d>] do_generic_file_read+0x80d/0xf90 [<ffffffff813cc955>] generic_file_read_iter+0x185/0x420 [...] [<ffffffff81510b06>] __vfs_read+0x256/0x3d0 [...] when get_init_ra_size() gets called from ondemand_readahead(). The net effect is that the initial readahead size is arch dependent for requested read lengths of zero: for example, since 1UL << (sizeof(unsigned long) * 8) evaluates to 1 on x86 while its result is 0 on ARMv7, the initial readahead size becomes 4 on the former and 0 on the latter. What's more, whether or not the file access timestamp is updated for zero length reads is decided differently for the two cases of IOCB_DIRECT being set or cleared: in the first case, generic_file_read_iter() explicitly skips updating that timestamp while in the latter case, it is always updated through the call to do_generic_file_read(). According to POSIX, zero length reads "do not modify the last data access timestamp" and thus, the IOCB_DIRECT behaviour is POSIXly correct. Let generic_file_read_iter() unconditionally check the requested read length at its entry and return immediately with success if it is zero. Signed-off-by: Nicolai Stange <nicstange@gmail.com> Cc: Al Viro <viro@zeniv.linux.org.uk> Reviewed-by: Jan Kara <jack@suse.cz> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 files changed, 4 insertions, 3 deletions
diff --git a/mm/filemap.c b/mm/filemap.c
index 7c00f105845e..a8c69c8c0a90 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1840,15 +1840,16 @@ generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
ssize_t retval = 0;
loff_t *ppos = &iocb->ki_pos;
loff_t pos = *ppos;
+ size_t count = iov_iter_count(iter);
+ if (!count)
+ goto out; /* skip atime */
if (iocb->ki_flags & IOCB_DIRECT) {
struct address_space *mapping = file->f_mapping;
struct inode *inode = mapping->host;
- size_t count = iov_iter_count(iter);
loff_t size;
- if (!count)
- goto out; /* skip atime */
size = i_size_read(inode);
retval = filemap_write_and_wait_range(mapping, pos,
pos + count - 1);

Privacy Policy