[PATCH next] shmem: minor fixes to splice-read implementation

[Hugh Dickins <hughd@google.com>]

generic_file_splice_read() makes a couple of preliminary checks (for
s_maxbytes and zero len), but shmem_file_splice_read() is called without
those: so check them inside it.  (But shmem does not support O_DIRECT,
so no need for that one here - and even if O_DIRECT support were stubbed
in, it would still just be using the page cache.)

HWPoison: my reading of folio_test_hwpoison() is that it only tests the
head page of a large folio, whereas splice_folio_into_pipe() will splice
as much of the folio as it can: so for safety we should also check the
has_hwpoisoned flag, set if any of the folio's pages are hwpoisoned.
(Perhaps that ugliness can be improved at the mm end later.)

The call to splice_zeropage_into_pipe() risked overrunning past EOF:
ask it for "part" not "len".

Fixes: b81d7b89becc ("shmem: Implement splice-read")
Signed-off-by: Hugh Dickins <hughd@google.com>
---
Thank you, David, for attending to tmpfs in your splice update:
yes, I too wish it could have just used the generic, but I'm sure
you're right that there's a number of reasons it needs its own.

 mm/shmem.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2902,6 +2902,11 @@ static ssize_t shmem_file_splice_read(struct file *in, loff_t *ppos,
 	loff_t isize;
 	int error = 0;
 
+	if (unlikely(*ppos >= MAX_LFS_FILESIZE))
+		return 0;
+	if (unlikely(!len))
+		return 0;
+
 	/* Work out how much data we can actually add into the pipe */
 	used = pipe_occupancy(pipe->head, pipe->tail);
 	npages = max_t(ssize_t, pipe->max_usage - used, 0);
@@ -2911,7 +2916,8 @@ static ssize_t shmem_file_splice_read(struct file *in, loff_t *ppos,
 		if (*ppos >= i_size_read(inode))
 			break;
 
-		error = shmem_get_folio(inode, *ppos / PAGE_SIZE, &folio, SGP_READ);
+		error = shmem_get_folio(inode, *ppos / PAGE_SIZE, &folio,
+					SGP_READ);
 		if (error) {
 			if (error == -EINVAL)
 				error = 0;
@@ -2920,7 +2926,9 @@ static ssize_t shmem_file_splice_read(struct file *in, loff_t *ppos,
 		if (folio) {
 			folio_unlock(folio);
 
-			if (folio_test_hwpoison(folio)) {
+			if (folio_test_hwpoison(folio) ||
+			    (folio_test_large(folio) &&
+			     folio_test_has_hwpoisoned(folio))) {
 				error = -EIO;
 				break;
 			}
@@ -2956,7 +2964,7 @@ static ssize_t shmem_file_splice_read(struct file *in, loff_t *ppos,
 			folio_put(folio);
 			folio = NULL;
 		} else {
-			n = splice_zeropage_into_pipe(pipe, *ppos, len);
+			n = splice_zeropage_into_pipe(pipe, *ppos, part);
 		}
 
 		if (!n)