git: a8c598de78fd - stable/15 - mpool/mpool_get.c: Avoid clobbering 'errno' when handling 'pread' errors

The branch stable/15 has been updated by bnovkov:

URL: https://cgit.FreeBSD.org/src/commit/?id=a8c598de78fd9ef6724d5f9366bc4b03fd5cbf9c

commit a8c598de78fd9ef6724d5f9366bc4b03fd5cbf9c
Author:     Bojan Novković <[email protected]>
AuthorDate: 2026-02-23 15:30:26 +0000
Commit:     Bojan Novković <[email protected]>
CommitDate: 2026-08-10 16:06:02 +0000

    mpool/mpool_get.c: Avoid clobbering 'errno' when handling 'pread' errors
    
    POSIX.1-2024 states that the 'free' function "shall not modify errno if
    ptr is a null pointer or a pointer previously returned as if by malloc()
    and not yet deallocated". However this is a fairly recent addition
    and non-compliant allocators might still clobber 'errno', causing
    'mpool_get' to return the wrong error code. Fix this by saving
    and restoring 'errno' after calling 'free'.
    
    Sponsored by:   Klara, Inc.
    Reviewed by:    obiwac
    Differential Revision:  https://reviews.freebsd.org/D55463
    MFC after:      1 week
    
    (cherry picked from commit bce0c14fe19defeef4f02cfebc018e9adf979783)
---
 lib/libc/db/mpool/mpool.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/lib/libc/db/mpool/mpool.c b/lib/libc/db/mpool/mpool.c
index 1bab66d73baf..0df19dc09e4d 100644
--- a/lib/libc/db/mpool/mpool.c
+++ b/lib/libc/db/mpool/mpool.c
@@ -214,10 +214,14 @@ mpool_get(MPOOL *mp, pgno_t pgno,
 	/* Read in the contents. */
 	off = mp->pagesize * pgno;
 	if ((nr = pread(mp->fd, bp->page, mp->pagesize, off)) != (ssize_t)mp->pagesize) {
+		int serrno;
+
 		switch (nr) {
 		case -1:
 			/* errno is set for us by pread(). */
+			serrno = errno;
 			free(bp);
+			errno = serrno;
 			mp->curcache--;
 			return (NULL);
 		case 0: