On 09/07/19 22:43, Andreas Dilger wrote: > On Jul 5, 2019, at 04:18, Jeff Layton <[email protected]> wrote: >> >> On Thu, 2019-07-04 at 21:18 +0000, Andreas Dilger wrote: >>> On Jul 4, 2019, at 04:43, Jeff Layton <[email protected]> wrote: >>>> On Thu, 2019-07-04 at 06:37 -0400, Jeff Layton wrote: >>>>> On Wed, 2019-07-03 at 20:24 +0000, Andreas Dilger wrote: >>>>>> When calling 'stat -c %N' to print the filename, don't explicitly >>>>>> request the size of the file via statx(), as it may add overhead on >>>>>> some filesystems. The size is only needed to optimize an allocation >>>>>> for the relatively rare case of reading a symlink name, and the worst >>>>>> effect is a somewhat-too-large temporary buffer may be allocated for >>>>>> areadlink_with_size(), or internal retries if buffer is too small. >>>>>> >>>>>> The file size will be returned by statx() on most filesystems, even >>>>>> if not requested, unless the filesystem considers this to be too >>>>>> expensive for that file, in which case the tradeoff is worthwhile. >>>>>> >>>>>> * src/stat.c: Don't explicitly request STATX_SIZE for filenames. >>>>>> Start with a 1KB buffer for areadlink_with_size() if st_size unset. >>>>>> --- >>>>>> src/stat.c | 6 ++++-- >>>>>> 1 file changed, 4 insertions(+), 2 deletions(-) >>>>>> >>>>>> diff --git a/src/stat.c b/src/stat.c >>>>>> index ec0bb7d..c887013 100644 >>>>>> --- a/src/stat.c >>>>>> +++ b/src/stat.c >>>>>> @@ -1282,7 +1282,7 @@ fmt_to_mask (char fmt) >>>>>> switch (fmt) >>>>>> { >>>>>> case 'N': >>>>>> - return STATX_MODE|STATX_SIZE; >>>>>> + return STATX_MODE; >>>>>> case 'd': >>>>>> case 'D': >>>>>> return STATX_MODE; >>>>>> @@ -1491,7 +1491,9 @@ print_stat (char *pformat, size_t prefix_len, >>>>>> unsigned int m, >>>>>> out_string (pformat, prefix_len, quoteN (filename)); >>>>>> if (S_ISLNK (statbuf->st_mode)) >>>>>> { >>>>>> - char *linkname = areadlink_with_size (filename, >>>>>> statbuf->st_size); >>>>>> + /* if statx() didn't set size, most symlinks are under 1KB */ >>>>>> + char *linkname = areadlink_with_size (filename, >>>>>> statbuf->st_size ?: >>>>>> + 1023); >>>>>> if (linkname == NULL) >>>>>> { >>>>>> error (0, errno, _("cannot read symbolic link %s"), >>>>> >>>>> Looks reasonable to me. >>>>> >>>>> Reviewed-by: Jeff Layton <[email protected]> >>>> >>>> Actually, I'll retract that just yet... >>>> >>>> I'm not sure that statbuf->st_size will be initialized to 0 if statx >>>> didn't fill out the stx_size field. You may need to accompany this patch >>>> with one that zeroes out the stx struct in do_stat. >>> >>> The kernel will explicitly zero out struct kstat at the beginning of the >>> callchain >>> so that only valid values filled in by the filesystem are returned, in >>> order to >>> avoid leaking any kernel memory to userspace: >>> >>> int vfs_getattr_nosec(const struct path *path, struct kstat *stat, >>> u32 request_mask, unsigned int query_flags) >>> { >>> struct inode *inode = d_backing_inode(path->dentry); >>> >>> memset(stat, 0, sizeof(*stat)); >>> stat->result_mask |= STATX_BASIC_STATS; >>> request_mask &= STATX_ALL; >>> query_flags &= KSTAT_QUERY_FLAGS; >>> >>> >>> Both the kernel generic_fillattr() and statx_to_stat() copy all of the >>> fields, >>> regardless of whether the corresponding bits are set in the request_mask or >>> not, >>> so this should be totally safe and there is no benefit to zero the struct in >>> userspace first. >> >> I don't think it's wise to rely on undocumented behavior. That could >> easily change in the future, and what if (e.g.) BSD grows a statx >> implementation that doesn't do that? >> >> I'd suggest at least zeroing out the stx_size field first. > > I looked at this, but there isn't much value to zero out the field since it > is unconditionally copied from the kernel upon return. For the unusual case > that the kernel doesn't touch the stx_size (or other) field then I've added > a zero initializer for the statx struct. Updated patch attached. > > Even so, it isn't a huge deal if the stx_size is random garbage. At worst it > will be too small and need a few readlink() iterations to get the right size, > and if too large it will be capped by areadlink_with_size(), so no harm is > done. With the separate changes to areadlink_with_size() to shrink the buffer > after allocation even this would not be significant.
Given the changes to areadlink_with_size() I've removed the 0 -> 1023 change in this patch, and just let areadlink_with_size() handle a 0 size appropriately. I'll push this later today. thanks, Pádraig
