From: Nicolai Stange Date: Fri, 25 Mar 2016 21:22:14 +0000 (-0700) Subject: mm/filemap: generic_file_read_iter(): check for zero reads unconditionally X-Git-Tag: v4.6-rc1~6^2~2 X-Git-Url: http://git.osdn.net/view?p=uclinux-h8%2Flinux.git;a=commitdiff_plain;h=e7080a439a6f507abbc860847c33efc39b5c1c6d 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: [...] [] ondemand_readahead+0x3aa/0x3d0 [] ? ondemand_readahead+0x3aa/0x3d0 [] ? find_get_entry+0x2d/0x210 [] page_cache_sync_readahead+0x63/0xa0 [] do_generic_file_read+0x80d/0xf90 [] generic_file_read_iter+0x185/0x420 [...] [] __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 Cc: Al Viro Reviewed-by: Jan Kara Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- 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);