[PATCH] use dynamic buffer for getmntent

[PATCH] use dynamic buffer for getmntent

[Natanael Copa]

overlayfs may have fairly long lines so we use getline to allocate a
buffer dynamically. The buffer will be allocated on first use, expand as
needed, but will never be free'ed.

Downstream bug: http://bugs.alpinelinux.org/issues/5703

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
---
 src/misc/mntent.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/src/misc/mntent.c b/src/misc/mntent.c
index a16d652..722a030 100644
--- a/src/misc/mntent.c
+++ b/src/misc/mntent.c
@@ -3,6 +3,11 @@
 #include <mntent.h>
 #include <errno.h>
 
+static char *internal_buf = NULL;
+static size_t internal_bufsize = 0;
+
+#define SENTINEL (char *)&internal_buf
+
 FILE *setmntent(const char *name, const char *mode)
 {
 	return fopen(name, mode);
@@ -16,13 +21,17 @@ int endmntent(FILE *f)
 
 struct mntent *getmntent_r(FILE *f, struct mntent *mnt, char *linebuf, int buflen)
 {
-	int cnt, n[8];
+	int cnt, n[8], use_internal = (linebuf == SENTINEL);
 
 	mnt->mnt_freq = 0;
 	mnt->mnt_passno = 0;
 
 	do {
-		fgets(linebuf, buflen, f);
+		if (use_internal) {
+			getline(&internal_buf, &internal_bufsize, f);
+			linebuf = internal_buf;
+		} else
+			fgets(linebuf, buflen, f);
 		if (feof(f) || ferror(f)) return 0;
 		if (!strchr(linebuf, '\n')) {
 			fscanf(f, "%*[^\n]%*[\n]");
@@ -49,9 +58,8 @@ struct mntent *getmntent_r(FILE *f, struct mntent *mnt, char *linebuf, int bufle
 
 struct mntent *getmntent(FILE *f)
 {
-	static char linebuf[256];
 	static struct mntent mnt;
-	return getmntent_r(f, &mnt, linebuf, sizeof linebuf);
+	return getmntent_r(f, &mnt, SENTINEL, 0);
 }
 
 int addmntent(FILE *f, const struct mntent *mnt)
-- 
2.10.0

Re: [PATCH] use dynamic buffer for getmntent

[Rich Felker]

On Thu, Sep 08, 2016 at 07:07:31PM +0200, Natanael Copa wrote:
> overlayfs may have fairly long lines so we use getline to allocate a
> buffer dynamically. The buffer will be allocated on first use, expand as
> needed, but will never be free'ed.
> 
> Downstream bug: http://bugs.alpinelinux.org/issues/5703
> 
> Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>

Thanks! Sorry for the delay. Committed with minor style fixes:

> ---
>  src/misc/mntent.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/src/misc/mntent.c b/src/misc/mntent.c
> index a16d652..722a030 100644
> --- a/src/misc/mntent.c
> +++ b/src/misc/mntent.c
> @@ -3,6 +3,11 @@
>  #include <mntent.h>
>  #include <errno.h>
>  
> +static char *internal_buf = NULL;
> +static size_t internal_bufsize = 0;

Explicit zero initialization is not needed, and current/going-forward
style in musl is not to use the NULL macro but rather 0 or (T*)0 if
needed (e.g. for variadic args).

> +
> +#define SENTINEL (char *)&internal_buf
> +
>  FILE *setmntent(const char *name, const char *mode)
>  {
>  	return fopen(name, mode);
> @@ -16,13 +21,17 @@ int endmntent(FILE *f)
>  
>  struct mntent *getmntent_r(FILE *f, struct mntent *mnt, char *linebuf, int buflen)
>  {
> -	int cnt, n[8];
> +	int cnt, n[8], use_internal = (linebuf == SENTINEL);
>  
>  	mnt->mnt_freq = 0;
>  	mnt->mnt_passno = 0;
>  
>  	do {
> -		fgets(linebuf, buflen, f);
> +		if (use_internal) {
> +			getline(&internal_buf, &internal_bufsize, f);
> +			linebuf = internal_buf;
> +		} else
> +			fgets(linebuf, buflen, f);

If if clause uses braces, else should too. There might be some
existing violations but I'm trying to make this consistent.

Rich