Fix (highly unlikely) race condition in readLink

Used to determine symlink size with stat and value with readlink.
This could technically result in garbage if symlink changed between
calls.  Also gets around the broken stat implementation in our
network filesystem (returns size + 1 giving a byte of garbage).

1 files changed, 11 insertions, 12 deletions

diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 9346d5dc4..16c2c97aa 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -216,18 +216,17 @@ bool pathExists(const Path & path)
 Path readLink(const Path & path)
 {
     checkInterrupt();
-    struct stat st = lstat(path);
-    if (!S_ISLNK(st.st_mode))
-        throw Error(format("'%1%' is not a symlink") % path);
-    auto bufSize = std::max(st.st_size, (off_t) PATH_MAX + 1);
-    char buf[bufSize];
-    ssize_t rlsize = readlink(path.c_str(), buf, bufSize);
-    if (rlsize == -1)
-        throw SysError(format("reading symbolic link '%1%'") % path);
-    else if (rlsize > bufSize)
-        throw Error(format("symbolic link '%1%' size overflow %2% > %3%")
-            % path % rlsize % bufSize);
-    return string(buf, rlsize);
+    for (ssize_t bufSize = PATH_MAX/4; true; bufSize += bufSize/2) {
+        char buf[bufSize];
+        ssize_t rlSize = readlink(path.c_str(), buf, bufSize);
+        if (rlSize == -1)
+            if (errno == EINVAL)
+                throw Error(format("'%1%' is not a symlink") % path);
+            else
+                throw SysError(format("reading symbolic link '%1%'") % path);
+        else if (rlSize < bufSize)
+            return string(buf, rlSize);
+    }
 }