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=4fea966734e1e6076595f53cd9b3b53b39ee1106'>root</a>/<a href='/guix/log/etc?id=4fea966734e1e6076595f53cd9b3b53b39ee1106'>etc</a>/<a href='/guix/log/etc/completion?id=4fea966734e1e6076595f53cd9b3b53b39ee1106'>completion</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/etc/completion?id=4fea966734e1e6076595f53cd9b3b53b39ee1106&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-09-24 17:53:02 +0200'>2020-09-24</span></td><td><a href='/guix/commit/etc/completion?id=8e0c5033b17fcf1e4596ee9a3d52157900acc6fa'>bash completion: Adjust to new 'guix help' output.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-09-21 23:33:49 +0200'>2020-09-21</span></td><td><a href='/guix/commit/etc/completion?id=92951bf46c42021f6f6521839c9c4ff2f88b8386'>bash completion: Complete file names after 'guix package -p'.</a><span class='msg-avail'>...</span></td><td>Fulbert</td></tr>
<tr><td><span title='2020-09-10 12:27:24 +0200'>2020-09-10</span></td><td><a href='/guix/commit/etc/completion?id=2ba81afcc204024255ce548e2c4cc4aed5541093'>bash completion: Restore completion for 'guix system', etc.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-06-27 23:42:20 +0200'>2020-06-27</span></td><td><a href='/guix/commit/etc/completion?id=4b4f890cb03d7c5b958fe08553dfad2f39a1d4f2'>bash completion: Complete 'guix environment' properly.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-06-04 23:26:43 +0200'>2020-06-04</span></td><td><a href='/guix/commit/etc/completion?id=538ca8dd65f0075cc65bcca18ed87f66a789290a'>bash completion: Complete sub-commands after 'guix time-machine'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-06-04 23:26:43 +0200'>2020-06-04</span></td><td><a href='/guix/commit/etc/completion?id=b7bb381ba3740d33652c2a8e7772c17b93b259cc'>bash completion: Complete file names after '-p' and '-C'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-05-13 09:30:26 +0300'>2020-05-13</span></td><td><a href='/guix/commit/etc/completion?id=3c986a7dc268756c097212de958f046b29b76c1b'>mailmap: Update entries for Nikita.</a><span class='msg-avail'>...</span></td><td>nikita</td></tr>
<tr><td><span title='2020-02-14 17:49:26 +0100'>2020-02-14</span></td><td><a href='/guix/commit/etc/completion?id=11415d35064cdba5cec1139aede18099cfa14547'>guix build: Add '--manifest' option.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>