See <http://lists.gnu.org/archive/html/bug-patch/2015-06/msg00009.html>

From 0507dfad5f060161f01840067e1bb1615257b636 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ludovic=20Court=C3=A8s?= <ludo@gnu.org>
Date: Mon, 8 Jun 2015 17:27:56 +0200
Subject: [PATCH] Do not rely on PATH_MAX when reading a symlink target.

* src/util.c (move_file, copy_file): Use 'fromst->st_size + 1' and
'tost->st_size + 1' for the allocation, and 'fromst->st_size' and
'tost->st_size' instead of PATH_MAX.  Fixes compilation on GNU/Hurd.
---
 src/util.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/util.c b/src/util.c
index 1cc08ba..67dbd3a 100644
--- a/src/util.c
+++ b/src/util.c
@@ -460,12 +460,12 @@ move_file (char const *from, bool *from_needs_removal,
 
 	  /* FROM contains the contents of the symlink we have patched; need
 	     to convert that back into a symlink. */
-	  char *buffer = xmalloc (PATH_MAX);
+	  char *buffer = xmalloc (fromst->st_size + 1);
 	  int fd, size = 0, i;
 
 	  if ((fd = safe_open (from, O_RDONLY | O_BINARY, 0)) < 0)
 	    pfatal ("Can't reopen file %s", quotearg (from));
-	  while ((i = read (fd, buffer + size, PATH_MAX - size)) > 0)
+	  while ((i = read (fd, buffer + size, fromst->st_size - size)) > 0)
 	    size += i;
 	  if (i != 0 || close (fd) != 0)
 	    read_fatal ();
@@ -610,10 +610,10 @@ copy_file (char const *from, char const *to, struct stat *tost,
 
   if (S_ISLNK (mode))
     {
-      char *buffer = xmalloc (PATH_MAX + 1);
+      char *buffer = xmalloc (tost->st_size + 1);
       ssize_t r;
 
-      if ((r = safe_readlink (from, buffer, PATH_MAX)) < 0)
+      if ((r = safe_readlink (from, buffer, tost->st_size)) < 0)
 	pfatal ("Can't read %s %s", "symbolic link", from);
       buffer[r] = '\0';
       if (safe_symlink (buffer, to) != 0)
-- 
2.16.2

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<div class='path'>path: <a href='/guix/log/?id=425df5b081eed7d2a0eadf1bf968af201714b613'>root</a>/<a href='/guix/log/nix?id=425df5b081eed7d2a0eadf1bf968af201714b613'>nix</a>/<a href='/guix/log/nix/libstore?id=425df5b081eed7d2a0eadf1bf968af201714b613'>libstore</a>/<a href='/guix/log/nix/libstore/builtins.cc?id=425df5b081eed7d2a0eadf1bf968af201714b613'>builtins.cc</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/nix/libstore/builtins.cc?id=425df5b081eed7d2a0eadf1bf968af201714b613&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2019-09-08 11:49:24 +0200'>2019-09-08</span></td><td><a href='/guix/commit/nix/libstore/builtins.cc?id=9fcc35c51fd579b3bb7b4f61cba6973675c3922c'>daemon: Run 'guix perform-download' directly.</a><span class='msg-avail'>...<span class='msg-tooltip'>* nix/scripts/download.in: Remove.
* nix/local.mk (nodist_pkglibexec_SCRIPTS): Remove 'scripts/download'.
* config-daemon.ac: Don't output 'nix/scripts/download'.
* nix/libstore/builtins.cc (builtinDownload): Invoke 'guix
perform-download' directly.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-02-04 16:20:01 +0100'>2019-02-04</span></td><td><a href='/guix/commit/nix/libstore/builtins.cc?id=959eaa1eb834fd47ceab3c5bbbe4967e808d530b'>daemon: Add "/guix" to default 'nixLibexecDir'.</a><span class='msg-avail'>...<span class='msg-tooltip'>This makes it easier to run the uninstalled daemon.

* nix/local.mk (libstore_a_CPPFLAGS): Append "/guix" to
NIX_LIBEXEC_DIR.
* build-aux/pre-inst-env.in (NIX_LIBEXEC_DIR): Adjust comment.
* nix/libstore/builtins.cc (builtinDownload): Remove SUBDIR and its
use.
* nix/libstore/local-store.cc (runAuthenticationProgram): Ditto.
* nix/libstore/gc.cc (addAdditionalRoots): Remove "/guix" prefix.
* nix/nix-daemon/guix-daemon.cc (main): Ditto.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-09-27 23:21:53 +0200'>2018-09-27</span></td><td><a href='/guix/commit/nix/libstore/builtins.cc?id=240a9c69a6064544a616acc521c993542c364948'>perform-download: Optionally report a "download-progress" trace.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/scripts/perform-download.scm (perform-download): Add
 #:print-build-trace? and pass it to 'url-fetch'.
(guix-perform-download): Define 'print-build-trace?' and pass it to
'perform-download'.
* guix/build/download.scm (ftp-fetch): Add #:print-build-trace? and
honor it.
(url-fetch): Likewise.
* nix/libstore/builtins.cc (builtinDownload): Set _NIX_OPTIONS
environment variable.
</span></span></td><td>Ludovic Courtès</td></tr>