git: 647f02d68a60 - stable/13 - libc __sfvwrite(): roll back FILE buffer pointer on fflush error

The branch stable/13 has been updated by markj:

URL: https://cgit.FreeBSD.org/src/commit/?id=647f02d68a60b66d063a15feed3c817966114d52

commit 647f02d68a60b66d063a15feed3c817966114d52
Author:     Konstantin Belousov <kib@FreeBSD.org>
AuthorDate: 2022-03-06 08:59:39 +0000
Commit:     Mark Johnston <markj@FreeBSD.org>
CommitDate: 2022-03-14 14:10:37 +0000

    libc  __sfvwrite(): roll back FILE buffer pointer on fflush error
    
    __sfvwrite() advances the pointer before calling fflush.  If fflush()
    fails, it is not enough to roll back inside it, because we cannot know
    how much was advanced by the caller.
    
    Reported by:    Peter <pmc@citylink.dinoex.sub.org>
    Reviewed by:    markj
    Sponsored by:   The FreeBSD Foundation
    Fixes:  86a16ada1ea608408cec370171d9f59353e97c77
    
    (cherry picked from commit bafaa70b6f9098d83d074968c8e6747ecec1e118)
---
 lib/libc/stdio/fvwrite.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/lib/libc/stdio/fvwrite.c b/lib/libc/stdio/fvwrite.c
index 50b32b8eca6e..2a161859afa9 100644
--- a/lib/libc/stdio/fvwrite.c
+++ b/lib/libc/stdio/fvwrite.c
@@ -54,6 +54,7 @@ int
 __sfvwrite(FILE *fp, struct __suio *uio)
 {
 	size_t len;
+	unsigned char *old_p;
 	char *p;
 	struct __siov *iov;
 	int w, s;
@@ -137,8 +138,12 @@ __sfvwrite(FILE *fp, struct __suio *uio)
 				COPY(w);
 				/* fp->_w -= w; */ /* unneeded */
 				fp->_p += w;
-				if (__fflush(fp))
+				old_p = fp->_p;
+				if (__fflush(fp) == EOF) {
+					if (old_p == fp->_p)
+						fp->_p -= w;
 					goto err;
+				}
 			} else if (len >= (w = fp->_bf._size)) {
 				/* write directly */
 				w = _swrite(fp, p, w);
@@ -177,8 +182,12 @@ __sfvwrite(FILE *fp, struct __suio *uio)
 				COPY(w);
 				/* fp->_w -= w; */
 				fp->_p += w;
-				if (__fflush(fp))
+				old_p = fp->_p;
+				if (__fflush(fp) == EOF) {
+					if (old_p == fp->_p)
+						fp->_p -= w;
 					goto err;
+				}
 			} else if (s >= (w = fp->_bf._size)) {
 				w = _swrite(fp, p, w);
 				if (w <= 0)