From c8cb6bcdf009e94c12c6e256b8e24a9bc5fdaf05 Mon Sep 17 00:00:00 2001
From: Rich Felker <dalias@aerifal.cx>
Date: Wed, 24 Oct 2012 23:16:41 -0400
Subject: correct locking in stdio functions that tried to be lock-free

these functions must behave as if they obtain the lock via flockfile
to satisfy POSIX requirements. since another thread can provably hold
the lock when they are called, they must wait to obtain the lock
before they can return, even if the correct return value could be
obtained without locking. in the case of fclose and freopen, failure
to do so could cause correct (albeit obscure) programs to crash or
otherwise misbehave; in the case of feof, ferror, and fwide, failure
to obtain the lock could sometimes return incorrect results. in any
case, having these functions proceed and return while another thread
held the lock was wrong.
---
 src/stdio/fclose.c  |  7 +++++--
 src/stdio/feof.c    |  5 ++++-
 src/stdio/ferror.c  |  5 ++++-
 src/stdio/fileno.c  |  5 +++++
 src/stdio/freopen.c | 24 ++++++++++++++----------
 src/stdio/fwide.c   |  6 ++++--
 6 files changed, 36 insertions(+), 16 deletions(-)

diff --git a/src/stdio/fclose.c b/src/stdio/fclose.c
index 8fdc3f7d..92bf7ff8 100644
--- a/src/stdio/fclose.c
+++ b/src/stdio/fclose.c
@@ -3,9 +3,12 @@
 int fclose(FILE *f)
 {
 	int r;
-	int perm = f->flags & F_PERM;
+	int perm;
+	
+	/* This lock is not paired with any unlock. */
+	FLOCK(f);
 
-	if (!perm) {
+	if (!(perm = f->flags & F_PERM)) {
 		OFLLOCK();
 		if (f->prev) f->prev->next = f->next;
 		if (f->next) f->next->prev = f->prev;
diff --git a/src/stdio/feof.c b/src/stdio/feof.c
index 5d7f4b02..56da6b91 100644
--- a/src/stdio/feof.c
+++ b/src/stdio/feof.c
@@ -4,7 +4,10 @@
 
 int feof(FILE *f)
 {
-	return !!(f->flags & F_EOF);
+	FLOCK(f);
+	int ret = !!(f->flags & F_EOF);
+	FUNLOCK(f);
+	return ret;
 }
 
 weak_alias(feof, feof_unlocked);
diff --git a/src/stdio/ferror.c b/src/stdio/ferror.c
index 8288a93d..d692eed9 100644
--- a/src/stdio/ferror.c
+++ b/src/stdio/ferror.c
@@ -4,7 +4,10 @@
 
 int ferror(FILE *f)
 {
-	return !!(f->flags & F_ERR);
+	FLOCK(f);
+	int ret = !!(f->flags & F_ERR);
+	FUNLOCK(f);
+	return ret;
 }
 
 weak_alias(ferror, ferror_unlocked);
diff --git a/src/stdio/fileno.c b/src/stdio/fileno.c
index 9ffb26d5..ba7f9391 100644
--- a/src/stdio/fileno.c
+++ b/src/stdio/fileno.c
@@ -2,6 +2,11 @@
 
 int fileno(FILE *f)
 {
+	/* f->fd never changes, but the lock must be obtained and released
+	 * anyway since this function cannot return while another thread
+	 * holds the lock. */
+	FLOCK(f);
+	FUNLOCK(f);
 	return f->fd;
 }
 
diff --git a/src/stdio/freopen.c b/src/stdio/freopen.c
index c80ce3b4..7ae116d8 100644
--- a/src/stdio/freopen.c
+++ b/src/stdio/freopen.c
@@ -4,8 +4,9 @@
  * hack the necessary parts of the new FILE into the old one, then
  * close the new FILE. */
 
-/* Locking is not necessary because, in the event of failure, the stream
- * passed to freopen is invalid as soon as freopen is called. */
+/* Locking IS necessary because another thread may provably hold the
+ * lock, via flockfile or otherwise, when freopen is called, and in that
+ * case, freopen cannot act until the lock is released. */
 
 int __dup3(int, int, int);
 
@@ -14,6 +15,8 @@ FILE *freopen(const char *restrict filename, const char *restrict mode, FILE *re
 	int fl = __fmodeflags(mode);
 	FILE *f2;
 
+	FLOCK(f);
+
 	fflush(f);
 
 	if (!filename) {
@@ -22,21 +25,22 @@ FILE *freopen(const char *restrict filename, const char *restrict mode, FILE *re
 		fl &= ~(O_CREAT|O_EXCL|O_CLOEXEC);
 		if (syscall(SYS_fcntl, f->fd, F_SETFL, fl) < 0)
 			goto fail;
-		return f;
 	} else {
 		f2 = fopen(filename, mode);
 		if (!f2) goto fail;
 		if (f2->fd == f->fd) f2->fd = -1; /* avoid closing in fclose */
 		else if (__dup3(f2->fd, f->fd, fl&O_CLOEXEC)<0) goto fail2;
-	}
 
-	f->flags = (f->flags & F_PERM) | f2->flags;
-	f->read = f2->read;
-	f->write = f2->write;
-	f->seek = f2->seek;
-	f->close = f2->close;
+		f->flags = (f->flags & F_PERM) | f2->flags;
+		f->read = f2->read;
+		f->write = f2->write;
+		f->seek = f2->seek;
+		f->close = f2->close;
 
-	fclose(f2);
+		fclose(f2);
+	}
+
+	FUNLOCK(f);
 	return f;
 
 fail2:
diff --git a/src/stdio/fwide.c b/src/stdio/fwide.c
index f4da47f6..48480685 100644
--- a/src/stdio/fwide.c
+++ b/src/stdio/fwide.c
@@ -5,6 +5,8 @@
 
 int fwide(FILE *f, int mode)
 {
-	if (!f->mode) f->mode = NORMALIZE(mode);
-	return f->mode;
+	FLOCK(f);
+	if (!f->mode) mode = f->mode = NORMALIZE(mode);
+	FUNLOCK(f);
+	return mode;
 }
-- 
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