[PATCH v3 0/2] util: Fix some problems in write_remainder()

[PATCH v3 0/2] util: Fix some problems in write_remainder()

[David Gibson]

With Markus Armbruster's help I re-examined the Coverity warning we've
had in this function for a while.  While I still don't understand some
steps in the report it's giving, I did spot some real problems in the
vicinity.  It turns out fixing those also fixes the Coverity warning,
so here we go.

Changes since v2:
 * Handle EINTR within write_all_buf() (suggested by Markus
   Armbruster)

David Gibson (2):
  util: Add helper to write() all of a buffer
  util: Remove possible quadratic behaviour from write_remainder()

 pcap.c |  3 +--
 util.c | 50 +++++++++++++++++++++++++++++++++++++++++---------
 util.h |  1 +
 3 files changed, 43 insertions(+), 11 deletions(-)

-- 
2.46.0

[PATCH v3 1/2] util: Add helper to write() all of a buffer

[David Gibson]

write(2) might not write all the data it is given.  Add a write_all_buf()
helper to keep calling it until all the given data is written, or we get an
error.

Currently we use write_remainder() to do this operation in pcap_frame().
That's a little awkward since it requires constructing an iovec, and future
changes we want to make to write_remainder() will be easier in terms of
this single buffer helper.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 pcap.c |  3 +--
 util.c | 25 +++++++++++++++++++++++++
 util.h |  1 +
 3 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/pcap.c b/pcap.c
index 46cc4b0d..e6b5ced4 100644
--- a/pcap.c
+++ b/pcap.c
@@ -86,9 +86,8 @@ static void pcap_frame(const struct iovec *iov, size_t iovcnt,
 		.caplen = l2len,
 		.len = l2len
 	};
-	struct iovec hiov = { &h, sizeof(h) };
 
-	if (write_remainder(pcap_fd, &hiov, 1, 0) < 0 ||
+	if (write_all_buf(pcap_fd, &h, sizeof(h)) < 0 ||
 	    write_remainder(pcap_fd, iov, iovcnt, offset) < 0)
 		debug_perror("Cannot log packet, length %zu", l2len);
 }
diff --git a/util.c b/util.c
index eede4e58..7db7c2e7 100644
--- a/util.c
+++ b/util.c
@@ -582,6 +582,31 @@ int do_clone(int (*fn)(void *), char *stack_area, size_t stack_size, int flags,
 #endif
 }
 
+/* write_all_buf() - write all of a buffer to an fd
+ * @fd:		File descriptor
+ * @buf:	Pointer to base of buffer
+ * @len:	Length of buffer
+ *
+ * Return: 0 on success, -1 on error (with errno set)
+ *
+ * #syscalls write
+ */
+int write_all_buf(int fd, const void *buf, size_t len)
+{
+	const char *p = buf;
+	size_t left = len;
+
+	while (left) {
+		ssize_t rc = write(fd, p, left);
+
+		if (rc < 0)
+			return -1;
+		p += rc;
+		left -= rc;
+	}
+	return 0;
+}
+
 /* write_remainder() - write the tail of an IO vector to an fd
  * @fd:		File descriptor
  * @iov:	IO vector
diff --git a/util.h b/util.h
index c7a59d5d..5e67f1fa 100644
--- a/util.h
+++ b/util.h
@@ -200,6 +200,7 @@ void pidfile_write(int fd, pid_t pid);
 int __daemon(int pidfile_fd, int devnull_fd);
 int fls(unsigned long x);
 int write_file(const char *path, const char *buf);
+int write_all_buf(int fd, const void *buf, size_t len);
 int write_remainder(int fd, const struct iovec *iov, size_t iovcnt, size_t skip);
 void close_open_files(int argc, char **argv);
 
-- 
@@ -200,6 +200,7 @@ void pidfile_write(int fd, pid_t pid);
 int __daemon(int pidfile_fd, int devnull_fd);
 int fls(unsigned long x);
 int write_file(const char *path, const char *buf);
+int write_all_buf(int fd, const void *buf, size_t len);
 int write_remainder(int fd, const struct iovec *iov, size_t iovcnt, size_t skip);
 void close_open_files(int argc, char **argv);
 
-- 
2.46.0

[PATCH v3 2/2] util: Remove possible quadratic behaviour from write_remainder()

[David Gibson]

write_remainder() steps through the buffers in an IO vector writing out
everything past a certain byte offset.  However, on each iteration it
rescans the buffer from the beginning to find out where we're up to.  With
an unfortunate set of write sizes this could lead to quadratic behaviour.

In an even less likely set of circumstances (total vector length > maximum
size_t) the 'skip' variable could overflow.  This is one factor in a
longstanding Coverity error we've seen (although I still can't figure out
the remainder of its complaint).

Rework write_remainder() to always work out our new position in the vector
relative to our old/current position, rather than starting from the
beginning each time.  As a bonus this seems to fix the Coverity error.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 util.c | 27 +++++++++++++++++----------
 1 file changed, 17 insertions(+), 10 deletions(-)

diff --git a/util.c b/util.c
index 7db7c2e7..87309c51 100644
--- a/util.c
+++ b/util.c
@@ -597,10 +597,15 @@ int write_all_buf(int fd, const void *buf, size_t len)
 	size_t left = len;
 
 	while (left) {
-		ssize_t rc = write(fd, p, left);
+		ssize_t rc;
+
+		do
+			rc = write(fd, p, left);
+		while ((rc < 0) && errno == EINTR);
 
 		if (rc < 0)
 			return -1;
+
 		p += rc;
 		left -= rc;
 	}
@@ -615,28 +620,30 @@ int write_all_buf(int fd, const void *buf, size_t len)
  *
  * Return: 0 on success, -1 on error (with errno set)
  *
- * #syscalls write writev
+ * #syscalls writev
  */
 int write_remainder(int fd, const struct iovec *iov, size_t iovcnt, size_t skip)
 {
-	size_t offset, i;
+	size_t i = 0, offset;
 
-	while ((i = iov_skip_bytes(iov, iovcnt, skip, &offset)) < iovcnt) {
+	while ((i += iov_skip_bytes(iov + i, iovcnt - i, skip, &offset)) < iovcnt) {
 		ssize_t rc;
 
 		if (offset) {
-			rc = write(fd, (char *)iov[i].iov_base + offset,
-				   iov[i].iov_len - offset);
-		} else {
-			rc = writev(fd, &iov[i], iovcnt - i);
+			/* Write the remainder of the partially written buffer */
+			if (write_all_buf(fd, (char *)iov[i].iov_base + offset,
+					  iov[i].iov_len - offset) < 0)
+				return -1;
+			i++;
 		}
 
+		/* Write as much of the remaining whole buffers as we can */
+		rc = writev(fd, &iov[i], iovcnt - i);
 		if (rc < 0)
 			return -1;
 
-		skip += rc;
+		skip = rc;
 	}
-
 	return 0;
 }
 
-- 
@@ -597,10 +597,15 @@ int write_all_buf(int fd, const void *buf, size_t len)
 	size_t left = len;
 
 	while (left) {
-		ssize_t rc = write(fd, p, left);
+		ssize_t rc;
+
+		do
+			rc = write(fd, p, left);
+		while ((rc < 0) && errno == EINTR);
 
 		if (rc < 0)
 			return -1;
+
 		p += rc;
 		left -= rc;
 	}
@@ -615,28 +620,30 @@ int write_all_buf(int fd, const void *buf, size_t len)
  *
  * Return: 0 on success, -1 on error (with errno set)
  *
- * #syscalls write writev
+ * #syscalls writev
  */
 int write_remainder(int fd, const struct iovec *iov, size_t iovcnt, size_t skip)
 {
-	size_t offset, i;
+	size_t i = 0, offset;
 
-	while ((i = iov_skip_bytes(iov, iovcnt, skip, &offset)) < iovcnt) {
+	while ((i += iov_skip_bytes(iov + i, iovcnt - i, skip, &offset)) < iovcnt) {
 		ssize_t rc;
 
 		if (offset) {
-			rc = write(fd, (char *)iov[i].iov_base + offset,
-				   iov[i].iov_len - offset);
-		} else {
-			rc = writev(fd, &iov[i], iovcnt - i);
+			/* Write the remainder of the partially written buffer */
+			if (write_all_buf(fd, (char *)iov[i].iov_base + offset,
+					  iov[i].iov_len - offset) < 0)
+				return -1;
+			i++;
 		}
 
+		/* Write as much of the remaining whole buffers as we can */
+		rc = writev(fd, &iov[i], iovcnt - i);
 		if (rc < 0)
 			return -1;
 
-		skip += rc;
+		skip = rc;
 	}
-
 	return 0;
 }
 
-- 
2.46.0

Re: [PATCH v3 0/2] util: Fix some problems in write_remainder()

[Stefano Brivio]

On Wed, 18 Sep 2024 20:44:04 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> With Markus Armbruster's help I re-examined the Coverity warning we've
> had in this function for a while.  While I still don't understand some
> steps in the report it's giving, I did spot some real problems in the
> vicinity.  It turns out fixing those also fixes the Coverity warning,
> so here we go.
> 
> Changes since v2:
>  * Handle EINTR within write_all_buf() (suggested by Markus
>    Armbruster)
> 
> David Gibson (2):
>   util: Add helper to write() all of a buffer
>   util: Remove possible quadratic behaviour from write_remainder()

Applied.

-- 
Stefano

Re: [PATCH v3 2/2] util: Remove possible quadratic behaviour from write_remainder()

[Markus Armbruster]

David Gibson <david@gibson.dropbear.id.au> writes:

> write_remainder() steps through the buffers in an IO vector writing out
> everything past a certain byte offset.  However, on each iteration it
> rescans the buffer from the beginning to find out where we're up to.  With
> an unfortunate set of write sizes this could lead to quadratic behaviour.
>
> In an even less likely set of circumstances (total vector length > maximum
> size_t) the 'skip' variable could overflow.  This is one factor in a
> longstanding Coverity error we've seen (although I still can't figure out
> the remainder of its complaint).
>
> Rework write_remainder() to always work out our new position in the vector
> relative to our old/current position, rather than starting from the
> beginning each time.  As a bonus this seems to fix the Coverity error.
>
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  util.c | 27 +++++++++++++++++----------
>  1 file changed, 17 insertions(+), 10 deletions(-)
>
> diff --git a/util.c b/util.c
> index 7db7c2e7..87309c51 100644
> --- a/util.c
> +++ b/util.c
> @@ -597,10 +597,15 @@ int write_all_buf(int fd, const void *buf, size_t len)
>  	size_t left = len;
>  
>  	while (left) {
> -		ssize_t rc = write(fd, p, left);
> +		ssize_t rc;
> +
> +		do
> +			rc = write(fd, p, left);
> +		while ((rc < 0) && errno == EINTR);
>  
>  		if (rc < 0)
>  			return -1;
> +
>  		p += rc;
>  		left -= rc;
>  	}

Uh, shouldn't this be squashed into PATCH 1?

> @@ -615,28 +620,30 @@ int write_all_buf(int fd, const void *buf, size_t len)
>   *
>   * Return: 0 on success, -1 on error (with errno set)
>   *
> - * #syscalls write writev
> + * #syscalls writev
>   */
>  int write_remainder(int fd, const struct iovec *iov, size_t iovcnt, size_t skip)
>  {
> -	size_t offset, i;
> +	size_t i = 0, offset;
>  
> -	while ((i = iov_skip_bytes(iov, iovcnt, skip, &offset)) < iovcnt) {
> +	while ((i += iov_skip_bytes(iov + i, iovcnt - i, skip, &offset)) < iovcnt) {
>  		ssize_t rc;
>  
>  		if (offset) {
> -			rc = write(fd, (char *)iov[i].iov_base + offset,
> -				   iov[i].iov_len - offset);
> -		} else {
> -			rc = writev(fd, &iov[i], iovcnt - i);
> +			/* Write the remainder of the partially written buffer */
> +			if (write_all_buf(fd, (char *)iov[i].iov_base + offset,
> +					  iov[i].iov_len - offset) < 0)
> +				return -1;
> +			i++;
>  		}
>  
> +		/* Write as much of the remaining whole buffers as we can */
> +		rc = writev(fd, &iov[i], iovcnt - i);
>  		if (rc < 0)
>  			return -1;
>  
> -		skip += rc;
> +		skip = rc;
>  	}
> -
>  	return 0;
>  }

Re: [PATCH v3 2/2] util: Remove possible quadratic behaviour from write_remainder()

[Stefano Brivio]

On Thu, 19 Sep 2024 12:42:58 +0200
Markus Armbruster <armbru@redhat.com> wrote:

> David Gibson <david@gibson.dropbear.id.au> writes:
> 
> > write_remainder() steps through the buffers in an IO vector writing out
> > everything past a certain byte offset.  However, on each iteration it
> > rescans the buffer from the beginning to find out where we're up to.  With
> > an unfortunate set of write sizes this could lead to quadratic behaviour.
> >
> > In an even less likely set of circumstances (total vector length > maximum
> > size_t) the 'skip' variable could overflow.  This is one factor in a
> > longstanding Coverity error we've seen (although I still can't figure out
> > the remainder of its complaint).
> >
> > Rework write_remainder() to always work out our new position in the vector
> > relative to our old/current position, rather than starting from the
> > beginning each time.  As a bonus this seems to fix the Coverity error.
> >
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  util.c | 27 +++++++++++++++++----------
> >  1 file changed, 17 insertions(+), 10 deletions(-)
> >
> > diff --git a/util.c b/util.c
> > index 7db7c2e7..87309c51 100644
> > --- a/util.c
> > +++ b/util.c
> > @@ -597,10 +597,15 @@ int write_all_buf(int fd, const void *buf, size_t len)
> >  	size_t left = len;
> >  
> >  	while (left) {
> > -		ssize_t rc = write(fd, p, left);
> > +		ssize_t rc;
> > +
> > +		do
> > +			rc = write(fd, p, left);
> > +		while ((rc < 0) && errno == EINTR);
> >  
> >  		if (rc < 0)
> >  			return -1;
> > +
> >  		p += rc;
> >  		left -= rc;
> >  	}  
> 
> Uh, shouldn't this be squashed into PATCH 1?

Oh, oops, in theory yes, but while reviewing this I just had a look at
the result in util.c, and now it's merged. It's not a big issue I'd
say. Thanks for reporting anyway.

-- 
Stefano