mm: fix mapping_seek_hole_data() overflow on last page

A local unprivileged process can create a shmem/tmpfs file with i_size ==
LLONG_MAX using memfd_create() and fallocate().  If the last page is
present in the page cache, lseek(SEEK_HOLE) on that page returns
0x8000000000000000 as a successful offset, which is LLONG_MIN when stored
in loff_t.

The same file has readable data at the last byte, but SEEK_DATA from that
offset returns ENXIO.

The overflow is in mapping_seek_hole_data():

  pos = round_up((u64)pos + 1, seek_size);

For the final page below LLONG_MAX, the next page boundary is
0x8000000000000000, which is then used as a signed file offset.  When
assigned to the loff_t pos, this overflows to LLONG_MIN, so a subsequent
"pos > end" comparison does not catch it.

Keep mapping_seek_hole_data() inside its documented [start, end) search
range: compute round_up() into a u64 variable and compare against (u64)end
so the overflow is detected, then clamp pos to end when the rounded-up
value goes past the search limit.

Link: https://lore.kernel.org/20260630125047.703170-1-yanzhen20011121@163.com
Signed-off-by: Zhen Yan <yanzhen20011121@163.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions