[musl] [PATCH v2] Fix UB in getmntent_r on extremely long lines

[Matthias Goergens <matthias.goergens@gmail.com>]

8974ef2124118e4ed8cad7ee0534b36e5c584c4e tried to fix mishandling of
extremely long lines.

Here's the relevant code snippet:

```
		len = strlen(linebuf);
		if (len > INT_MAX) continue;
		for (i = 0; i < sizeof n / sizeof *n; i++) n[i] = len;
		sscanf(linebuf, " %n%*s%n %n%*s%n %n%*s%n %n%*s%n %d %d",
			n, n+1, n+2, n+3, n+4, n+5, n+6, n+7,
			&mnt->mnt_freq, &mnt->mnt_passno);
	} while (linebuf[n[0]] == '#' || n[1]==len);
```

Alas, that introduced undefined behaviour: if the very first line
handled in the function is extremely long, `n` stays uninitialised, and
thus accessing `n[0]` and `n[1]` is UB.

If we handle a few sane lines before hitting a crazy long line, we don't
hit C-level undefined behaviour, but the function arguably still does
the wrong thing.

The documentation suggests that we could return NULL on failure, but
Rich Felker explained that skipping extremely long lines makes more
sense here.  So that's what we do.
---
 src/misc/mntent.c | 36 ++++++++++++++++++++++--------------
 1 file changed, 22 insertions(+), 14 deletions(-)

diff --git a/src/misc/mntent.c b/src/misc/mntent.c
index d404fbe3..cdd3d0fd 100644
--- a/src/misc/mntent.c
+++ b/src/misc/mntent.c
@@ -29,21 +29,29 @@ struct mntent *getmntent_r(FILE *f, struct mntent *mnt, char *linebuf, int bufle
 	mnt->mnt_passno = 0;
 
 	do {
-		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]");
-			errno = ERANGE;
-			return 0;
-		}
+		do {
+			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]");
+				errno = ERANGE;
+				return 0;
+			}
+			len = strlen(linebuf);
+			// In theory, with `use_internal` we could read a line longer than
+			// INT_MAX.  But we don't want to incentivise using the legacy
+			// thread-unsafe API (`getmntent`).
 
-		len = strlen(linebuf);
-		if (len > INT_MAX) continue;
+			// The thread-safe API of getmntent_r only supports lengths up to
+			// INT_MAX, because of `int buflen` in the function signature.
+
+			// As a compromise, we skip extremely long lines.
+		} while (len > INT_MAX);
 		for (i = 0; i < sizeof n / sizeof *n; i++) n[i] = len;
 		sscanf(linebuf, " %n%*s%n %n%*s%n %n%*s%n %n%*s%n %d %d",
 			n, n+1, n+2, n+3, n+4, n+5, n+6, n+7,
-- 
2.40.0