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Fix (highly unlikely) race condition in readLink

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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).
This commit is contained in:
Tyson Whitehead
2017-10-27 12:15:31 -04:00
parent 12991152be
commit 07d2c6d213
1 changed files with 11 additions and 12 deletions
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23
src/libutil/util.cc
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@@ -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);
}
}