From 07d2c6d213660e178fcdf128ea6f1f36d9d8a85d Mon Sep 17 00:00:00 2001 From: Tyson Whitehead Date: Fri, 27 Oct 2017 12:15:31 -0400 Subject: [PATCH] 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). --- src/libutil/util.cc | 23 +++++++++++------------ 1 file 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); + } }