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=2d04076332eedb7fcb292aa64b0dc18a53f58751'>root</a>/<a href='/guix/log/gnu?id=2d04076332eedb7fcb292aa64b0dc18a53f58751'>gnu</a>/<a href='/guix/log/gnu/packages?id=2d04076332eedb7fcb292aa64b0dc18a53f58751'>packages</a>/<a href='/guix/log/gnu/packages/gprolog.scm?id=2d04076332eedb7fcb292aa64b0dc18a53f58751'>gprolog.scm</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/gnu/packages/gprolog.scm?id=2d04076332eedb7fcb292aa64b0dc18a53f58751&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2018-09-20 15:27:43 -0400'>2018-09-20</span></td><td><a href='/guix/commit/gnu/packages/gprolog.scm?id=f5c10a94eff314ed985089719e817cd036a746f1'>gnu: gprolog: Update to 1.4.5.</a><span class='msg-avail'>...</span></td><td>fis trivial</td></tr>
<tr><td><span title='2017-09-20 20:58:04 -0400'>2017-09-20</span></td><td><a href='/guix/commit/gnu/packages/gprolog.scm?id=dc1d3cdef70f0e3c047c229c2a0e561d1f47bde8'>gnu: Use 'modify-phases' syntax.</a><span class='msg-avail'>...</span></td><td>Kei Kebreau</td></tr>
<tr><td><span title='2016-05-28 07:27:46 +0800'>2016-05-28</span></td><td><a href='/guix/commit/gnu/packages/gprolog.scm?id=1a6d3cd374bcff136592e04caa0dafada3352ecd'>gnu: gprolog: Don't install files into "$out/gprolog-1.4.4".</a><span class='msg-avail'>...</span></td><td>宋文武</td></tr>