From 538e4b2054e9802ebc860943e0a43baf2ee46741 Mon Sep 17 00:00:00 2001
From: Tavian Barnes <tavianator@tavianator.com>
Date: Tue, 31 Jan 2017 21:01:14 -0500
Subject: Don't trust st_size when reading symlinks

Linux /proc, for example, reports a st_size of 0 for everything.  It's
nice to be able to use -lname on them anyway.
---
 eval.c | 13 +------------
 util.c | 30 ++++++++++++++++++++++++++++++
 util.h | 13 +++++++++++++
 3 files changed, 44 insertions(+), 12 deletions(-)

diff --git a/eval.c b/eval.c
index 6a3da7a..4494cff 100644
--- a/eval.c
+++ b/eval.c
@@ -599,23 +599,12 @@ bool eval_lname(const struct expr *expr, struct eval_state *state) {
 		goto done;
 	}
 
-	size_t size = statbuf->st_size + 1;
-	name = malloc(size);
+	name = xreadlinkat(ftwbuf->at_fd, ftwbuf->at_path, statbuf->st_size);
 	if (!name) {
 		eval_error(state);
 		goto done;
 	}
 
-	ssize_t len = readlinkat(ftwbuf->at_fd, ftwbuf->at_path, name, size);
-	if (len < 0) {
-		eval_error(state);
-		goto done;
-	} else if (len >= size) {
-		goto done;
-	}
-
-	name[len] = '\0';
-
 	ret = fnmatch(expr->sdata, name, expr->idata) == 0;
 
 done:
diff --git a/util.c b/util.c
index ce0b458..f3f2c5f 100644
--- a/util.c
+++ b/util.c
@@ -29,6 +29,36 @@ int xreaddir(DIR *dir, struct dirent **de) {
 	}
 }
 
+char *xreadlinkat(int fd, const char *path, size_t size) {
+	++size; // NUL-terminator
+	ssize_t len;
+	char *name = NULL;
+
+	while (true) {
+		char *new_name = realloc(name, size);
+		if (!new_name) {
+			goto error;
+		}
+		name = new_name;
+
+		len = readlinkat(fd, path, name, size);
+		if (len < 0) {
+			goto error;
+		} else if (len >= size) {
+			size *= 2;
+		} else {
+			break;
+		}
+	}
+
+	name[len] = '\0';
+	return name;
+
+error:
+	free(name);
+	return NULL;
+}
+
 bool isopen(int fd) {
 	return fcntl(fd, F_GETFD) >= 0 || errno != EBADF;
 }
diff --git a/util.h b/util.h
index 4fd5962..8fa775d 100644
--- a/util.h
+++ b/util.h
@@ -48,6 +48,19 @@
  */
 int xreaddir(DIR *dir, struct dirent **de);
 
+/**
+ * readlinkat() wrapper that dynamically allocates the result.
+ *
+ * @param fd
+ *         The base directory descriptor.
+ * @param path
+ *         The path to the link, relative to fd.
+ * @param size
+ *         An estimate for the size of the link name (pass 0 if unknown).
+ * @return The target of the link, allocated with malloc(), or NULL on failure.
+ */
+char *xreadlinkat(int fd, const char *path, size_t size);
+
 /**
  * Check if a file descriptor is open.
  */
-- 
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