be able to break inheritance of LD_LIBRARY_PATH

[u-igbb@aetey.se]

Hello,

I was aware of musl for some time and now consider deploying it as
a default library for new software builds, due to its very appealing
virtues.

Yet there is a small but important issue.

For our software setup it is crucial (quite useful otherwise in general)
to be able to specify the location of the dynamic libraries per binary/run
_without_ the unconditional inheritance imposed by LD_LIBRARY_PATH.

A very nice solution would be the ability to explicitely run a standalone
dynamic loader, as implemented in both glibc and uclibc. We are heavily
relying on this functionality.

I do not know how hard it would be to teach the musl loader
to be runnable standalone and which corner cases this might create.

As a simpler approach I might suggest simply being able to drop
LD_LIBRARY_PATH as soon as it has been read. An extra environment
variable as a flag would do.

Compared to a standalone loader this lacks the ability to run
a binary with a different version of the loader/musl but at least
makes it straightforward and safe to freely specify where to find other
libraries.

A naïve implementation might look as follows:

--- src/ldso/dynlink.c.ori      2014-03-28 10:37:34.821317811 +0100
+++ src/ldso/dynlink.c  2014-03-28 11:21:16.828047766 +0100
@@ -962,6 +962,7 @@
        size_t vdso_base;
        size_t *auxv;
        char **envp = argv+argc+1;
+       int forget_ld_library_path = 0;
 
        /* Find aux vector just past environ[] */
        for (i=argc+1; argv[i]; i++)
@@ -969,8 +970,19 @@
                        env_path = argv[i]+16;
                else if (!memcmp(argv[i], "LD_PRELOAD=", 11))
                        env_preload = argv[i]+11;
+               else if (!memcmp(argv[i], "FORGET_LD_LIBRARY_PATH=", 23))
+                       forget_ld_library_path = 1;
        auxv = (void *)(argv+i+1);
 
+       /* one _may_ wish to break the inheritance of LD_LIBRARY_PATH,
+        * the hack below only works if the corresponding memory is writable
+        * -- rl */
+       if (forget_ld_library_path)
+               for (i=argc+1; argv[i]; i++)
+                       if (!memcmp(argv[i], "LD_LIBRARY_PATH=", 16) ||
+                           !memcmp(argv[i], "FORGET_LD_LIBRARY_PATH=", 23))
+                               argv[i][0] = 'X';
+
        decode_vec(auxv, aux, AUX_CNT);
 
        /* Only trust user/env if kernel says we're not suid/sgid */


What do you think about this? Can this or something better be done?
I would love to be able to go with musl.

Regards,
Rune