From df7dd7127c4a0db0583668cefa72bc5f2c2e088c Mon Sep 17 00:00:00 2001
From: Jonathan Wakely <jwakely@redhat.com>
Date: Fri, 5 Jan 2018 21:27:20 +0000
Subject: [PATCH] PR libstdc++/79283 fix filesystem::read_symlink for /proc

Backport from mainline
2017-10-25  Jonathan Wakely  <jwakely@redhat.com>

	PR libstdc++/79283
	* src/filesystem/ops.cc (read_symlink): Handle st_size being zero.

From-SVN: r256286
---
 libstdc++-v3/ChangeLog             |  8 +++++++
 libstdc++-v3/src/filesystem/ops.cc | 37 ++++++++++++++++++++++--------
 2 files changed, 36 insertions(+), 9 deletions(-)

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 68c56b10207..be33fd264c0 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,11 @@
+2018-01-05  Jonathan Wakely  <jwakely@redhat.com>
+
+	Backport from mainline
+	2017-10-25  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/79283
+	* src/filesystem/ops.cc (read_symlink): Handle st_size being zero.
+
 2018-01-04  Ville Voutilainen  <ville.voutilainen@gmail.com>
 
 	Backport from mainline
diff --git a/libstdc++-v3/src/filesystem/ops.cc b/libstdc++-v3/src/filesystem/ops.cc
index c711de16f06..8c403263a4b 100644
--- a/libstdc++-v3/src/filesystem/ops.cc
+++ b/libstdc++-v3/src/filesystem/ops.cc
@@ -1199,26 +1199,45 @@ fs::read_symlink(const path& p)
 
 fs::path fs::read_symlink(const path& p, error_code& ec)
 {
+  path result;
 #ifdef _GLIBCXX_HAVE_SYS_STAT_H
   stat_type st;
   if (::lstat(p.c_str(), &st))
     {
       ec.assign(errno, std::generic_category());
-      return {};
+      return result;
     }
-  std::string buf(st.st_size, '\0');
-  ssize_t len = ::readlink(p.c_str(), &buf.front(), buf.size());
-  if (len == -1)
+  std::string buf(st.st_size ? st.st_size + 1 : 128, '\0');
+  do
     {
-      ec.assign(errno, std::generic_category());
-      return {};
+      ssize_t len = ::readlink(p.c_str(), &buf.front(), buf.size());
+      if (len == -1)
+	{
+	  ec.assign(errno, std::generic_category());
+	  return result;
+	}
+      else if (len == (ssize_t)buf.size())
+	{
+	  if (buf.size() > 4096)
+	    {
+	      ec.assign(ENAMETOOLONG, std::generic_category());
+	      return result;
+	    }
+	  buf.resize(buf.size() * 2);
+	}
+      else
+	{
+	  buf.resize(len);
+	  result.assign(buf);
+	  ec.clear();
+	  break;
+	}
     }
-  ec.clear();
-  return path{buf.data(), buf.data()+len};
+  while (true);
 #else
   ec = std::make_error_code(std::errc::not_supported);
-  return {};
 #endif
+  return result;
 }