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fstat(2) after open(2) for set -C case when initial stat(2) was !S_ISREG

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to fix race condition as suggested by jilles
This commit is contained in:
tg
2015-12-31 21:03:47 +00:00
parent c56d848a10
commit 4059e105a2
3 changed files with 37 additions and 15 deletions
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42
exec.c
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@ -23,7 +23,7 @@
#include "sh.h"
__RCSID("$MirOS: src/bin/mksh/exec.c,v 1.169 2015/12/31 12:58:43 tg Exp $");
__RCSID("$MirOS: src/bin/mksh/exec.c,v 1.170 2015/12/31 21:03:47 tg Exp $");
#ifndef MKSH_DEFAULT_EXECSHELL
#define MKSH_DEFAULT_EXECSHELL MKSH_UNIXROOT "/bin/sh"
@ -1363,7 +1363,7 @@ iosetup(struct ioword *iop, struct tbl *tp)
int u = -1;
char *cp = iop->ioname;
int iotype = iop->ioflag & IOTYPE;
bool do_open = true, do_close = false;
bool do_open = true, do_close = false, do_fstat = false;
int flags = 0;
struct ioword iotmp;
struct stat statb;
@ -1392,14 +1392,27 @@ iosetup(struct ioword *iop, struct tbl *tp)
break;
case IOWRITE:
flags = O_WRONLY | O_CREAT | O_TRUNC;
/*
* The stat() is here to allow redirections to
* things like /dev/null without error.
*/
if (Flag(FNOCLOBBER) && !(iop->ioflag & IOCLOB) &&
(stat(cp, &statb) < 0 || S_ISREG(statb.st_mode)))
flags |= O_EXCL;
if (Flag(FNOCLOBBER) && !(iop->ioflag & IOCLOB)) {
/* >file under set -C */
if (stat(cp, &statb)) {
/* nonexistent file */
flags = O_WRONLY | O_CREAT | O_EXCL;
} else if (S_ISREG(statb.st_mode)) {
/* regular file, refuse clobbering */
goto clobber_refused;
} else {
/*
* allow redirections to things
* like /dev/null without error
*/
flags = O_WRONLY;
/* but check again after opening */
do_fstat = true;
}
} else {
/* >|file or set +C */
flags = O_WRONLY | O_CREAT | O_TRUNC;
}
break;
case IORDWR:
@ -1444,6 +1457,15 @@ iosetup(struct ioword *iop, struct tbl *tp)
return (-1);
}
u = binopen3(cp, flags, 0666);
if (do_fstat && u >= 0) {
/* prevent race conditions */
if (fstat(u, &statb) || S_ISREG(statb.st_mode)) {
close(u);
clobber_refused:
u = -1;
errno = EEXIST;
}
}
}
if (u < 0) {
/* herein() may already have printed message */