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=75bdd4b05253c0e6ca5399f60e424f0f00fdb673'>root</a>/<a href='/guix/log/tests?id=75bdd4b05253c0e6ca5399f60e424f0f00fdb673'>tests</a>/<a href='/guix/log/tests/guix-build-branch.sh?id=75bdd4b05253c0e6ca5399f60e424f0f00fdb673'>guix-build-branch.sh</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/tests/guix-build-branch.sh?id=75bdd4b05253c0e6ca5399f60e424f0f00fdb673&amp<tr><td><span title='2022-08-09 15:16:06 +0200'>2022-08-09</span></td><td><a href='/guix/commit/etc/system-tests.scm?id=5bce4c82422de6beb3ce6120ba1592be898c2b72'>build-system: Add 'channel-build-system'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/ci.scm (channel-build-system, channel-source-&gt;package): Remove.
* gnu/packages/package-management.scm (channel-source-&gt;package): New
procedure, moved from (gnu ci).
* guix/build-system/channel.scm: New file, with code moved from (gnu ci).
* doc/guix.texi (Build Systems): Document it.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2022-07-10 23:57:14 +0200'>2022-07-10</span></td><td><a href='/guix/commit/etc/system-tests.scm?id=9fdc4b6c283c5aa5cf10205d87fb2c58b829b9d0'>monads: Add 'mparameterize'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* etc/system-tests.scm (mparameterize): Move to...
* guix/monads.scm (mparameterize): ... here.
* tests/monads.scm ("mparameterize"): New test.
* .dir-locals.el (c-mode): Add it.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2022-04-11 00:25:33 +0200'>2022-04-11</span></td><td><a href='/guix/commit/etc/system-tests.scm?id=dbde386794cb5f4773b94a20ef585ca0f881544a'>tests: System tests really parameterize 'current-guix-package'.</a><span class='msg-avail'>...<span class='msg-tooltip'>Until now, 'current-guix-package' was parameterized in the wrong
context.  Thus, 'current-guix' would end up building a variant of the
'guix' package instead of the package returned by
'channel-source-&gt;package', which is much less expensive to build.

* etc/system-tests.scm (mparameterize): New macro.
(tests-for-current-guix): Change the 'value' field of each &lt;system-test&gt;
record to parameterize 'current-guix-package' for good.
</span></span></td><td>Ludovic Courtès</td></tr>