Allow users to specify that they don't want symlink target path expansion

NEWS.md

@@ -4,6 +4,11 @@ UNRELEASED
 
 * The `chroot`(8) wrapper and `fakechroot`(1) command can work with POSIX
   shell.
+* The `fakechroot`(1) command now supports an environment variable called
+  `FAKECHROOT_NOEXPAND_SYMLINK_TARGET`, which disables path expansion on
+  the targets of symlink syscalls. This more closely resmebles traditional
+  `chroot` behavior, which treats the target of a symlink as an opaque
+  string rather than a path.
 
 ## Version 2.19
 

man/fakechroot.pod

@@ -302,6 +302,16 @@ to make debootstrap(8) working correctly.
 To prevent some looping, the command substitution is done only if
 C<FAKECHROOT_CMD_ORIG> variable is not set currently.
 
+=item B<FAKECHROOT_NOEXPAND_SYMLINK_TARGET>
+
+Fakechroot defaults to expanding the target of a symlink so that any absolute
+symlinks created inside a fakechroot session will continue to work outside of
+it. If that behavior is undesirable, setting this environment variable will
+cause the symlink and symlinkat syscalls to create symlinks with the raw target
+paths requested by the invoker. Note that this does not affect the path of the
+symlink being created in any way, which still gets expanded according to the
+usual chroot rules.
+
 =item B<LD_LIBRARY_PATH>, B<LD_PRELOAD>
 
 Fakechroot is implemented by wrapping system calls. This is accomplished by

src/libfakechroot.c

@@ -59,6 +59,7 @@ char *preserve_env_list[] = {
     "FAKECHROOT_ELFLOADER_OPT_ARGV0",
     "FAKECHROOT_EXCLUDE_PATH",
     "FAKECHROOT_LDLIBPATH",
+    "FAKECHROOT_NOEXPAND_SYMLINK_TARGET",
     "FAKECHROOT_VERSION",
     "FAKEROOTKEY",
     "FAKED_MODE",

src/symlink.c

@@ -27,7 +27,10 @@ wrapper(symlink, int, (const char * oldpath, const char * newpath))
 {
     char tmp[FAKECHROOT_PATH_MAX];
     debug("symlink(\"%s\", \"%s\")", oldpath, newpath);
-    expand_chroot_rel_path(oldpath);
+    char *noexpand_symlink_target = getenv("FAKECHROOT_NOEXPAND_SYMLINK_TARGET");
+    if (!noexpand_symlink_target)
+        expand_chroot_rel_path(oldpath);
+
     strcpy(tmp, oldpath);
     oldpath = tmp;
     expand_chroot_path(newpath);

src/symlinkat.c

@@ -30,7 +30,10 @@ wrapper(symlinkat, int, (const char * oldpath, int newdirfd, const char * newpat
 {
     char tmp[FAKECHROOT_PATH_MAX];
     debug("symlinkat(\"%s\", %d, \"%s\")", oldpath, newdirfd, newpath);
-    expand_chroot_rel_path(oldpath);
+    char *noexpand_symlink_target = getenv("FAKECHROOT_NOEXPAND_SYMLINK_TARGET");
+    if (!noexpand_symlink_target)
+        expand_chroot_rel_path(oldpath);
+
     strcpy(tmp, oldpath);
     oldpath = tmp;
     expand_chroot_path_at(newdirfd, newpath);