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/*
* Copyright (c) 2017 William Pitcock <nenolod@dereferenced.org>
* Copyright (c) 2018 Samuel Holland <samuel@sholland.org>
*
* Permission to use, copy, modify, and/or distribute this software for any
* purpose with or without fee is hereby granted, provided that the above
* copyright notice and this permission notice appear in all copies.
*
* This software is provided 'as is' and without any warranty, express or
* implied. In no event shall the authors be liable for any damages arising
* from the use of this software.
*/
#include <limits.h> /* PATH_MAX */
#include <stddef.h> /* NULL */
#include <stdio.h> /* fputs, fwrite, stderr */
#include <stdlib.h> /* calloc, EXIT_FAILURE, getenv */
#include <string.h> /* strcpy, strlcpy */
#include <unistd.h> /* execve, ssize_t */
#ifndef PATH_MAX
#define PATH_MAX 16384
#endif
#ifndef LIBGCOMPAT
#error LIBGCOMPAT must be defined
#endif
#ifndef LINKER
#error LINKER must be defined
#endif
#ifndef LOADER
#error LOADER must be defined
#endif
/*
* Given the argv { "foo", "bar", "baz" }, and the environment variable
* "LD_PRELOAD=/another/preload.so", the new argv will be {
* "ld-linux-$ARCH.so.N",
* "--argv0",
* "foo",
* "--preload",
* "/path/to/libgcompat.so /another/preload.so",
* "/path/to/foo",
* "bar",
* "baz"
* }.
*
* NOTE: The new argv needs 6 more entries than the original argc: five for
* arguments added at the beginning, and one for the NULL sentinel at the end
* not included in argc. Because calloc() is used, the sentinel is not
* explicitly set on its own, but *it is still there*!
*
* NOTE: The name given in argv[0] to musl *must* match glibc's ld.so filename.
* Many glibc-linked programs have an explicit dependency on its ld.so, but the
* file placed at that path (this loader) is not a real shared library. If musl
* tries to load this file as a shared library, it will fail. Thankfully, since
* musl treats argv[0] as "its" name when run directly, it considers a library
* by that name to already be loaded, and ignores the problematic dependency.
*
* NOTE: LD_PRELOAD entries are processed in order, and the environment variable
* is ignored if the parameter is given. In case the glibc-linked program needs
* another preloaded library, it should be appended to the argument so it gets
* loaded after libgcompat. Leave the environment variable as is, so if the
* program runs other glibc-linked programs, the same transformation gets
* applied there as well.
*
* NOTE: In order for the program to be able to re-exec itself it needs the
* fully-resolved path from /proc/self/exe ("target" below). This path is given
* to musl on the command line in new_argv[5], but it is not directly available
* to the program. An intercepted readlink("/proc/self/exe") must read it from
* /proc/self/cmdline. Thus, if its position in new_argv changes, that function
* must also be updated to match.
*/
int main(int argc, char *argv[], char *envp[])
{
char **new_argv = calloc(argc + 6, sizeof(char *));
char preload[PATH_MAX];
char target[PATH_MAX];
ssize_t i, j, len;
strcpy(preload, LIBGCOMPAT " ");
if (getenv("LD_PRELOAD") != NULL) {
len = strlcat(preload, getenv("LD_PRELOAD"), sizeof(preload));
if ((size_t) len >= sizeof(preload)) {
fputs("too many preloaded libraries", stderr);
return EXIT_FAILURE;
}
}
len = readlink("/proc/self/exe", target, sizeof(target));
if (len < 0 || len == sizeof(target)) {
perror("readlink");
return EXIT_FAILURE;
}
target[len] = '\0';
new_argv[0] = LOADER;
new_argv[1] = "--argv0";
new_argv[2] = argv[0];
new_argv[3] = "--preload";
new_argv[4] = preload;
new_argv[5] = target;
for (i = 6, j = 1; j < argc; ++i, ++j) {
new_argv[i] = argv[j];
}
execve(LINKER, new_argv, envp);
/* execve only returns if there is an error. */
perror("execve");
return EXIT_FAILURE;
}
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