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/*
* Copyright (c) 2021 Andrew G. Morgan <morgan@kernel.org>
*
* Some header magic to help make a shared object run-able as a stand
* alone executable binary.
*
* This is a slightly more sophisticated implementation than the
* answer I posted here:
*
* https://stackoverflow.com/a/68339111/14760867
*
* Compile your shared library with:
*
* -DSHARED_LOADER="\"ld-linux...\"" (loader for your target system)
* ...
* --entry=__so_start
*/
#include <stdlib.h>
#include <string.h>
#ifdef __EXECABLE_H
#error "only include execable.h once"
#endif
#define __EXECABLE_H
#ifdef __GLIBC__
/*
* https://bugzilla.kernel.org/show_bug.cgi?id=219880 So far as I can
* tell this value is some legacy magic meaning, but is a detail no
* longer important to glibc. Only the existence of this constant in
* the linkage is needed.
*/
extern const int _IO_stdin_used;
const int _IO_stdin_used __attribute__((weak)) = 131073;
#endif /* def __GLIBC__ */
const char __execable_dl_loader[] __attribute((section(".interp"))) =
SHARED_LOADER ;
static void __execable_parse_args(int *argc_p, char ***argv_p)
{
int argc = 0;
char **argv = NULL;
FILE *f = fopen("/proc/self/cmdline", "rb");
if (f != NULL) {
char *mem = NULL, *p;
size_t size = 32, offset;
for (offset=0; ; size *= 2) {
char *new_mem = realloc(mem, size+1);
if (new_mem == NULL) {
perror("unable to parse arguments");
fclose(f);
if (mem != NULL) {
free(mem);
}
exit(1);
}
mem = new_mem;
offset += fread(mem+offset, 1, size-offset, f);
if (offset < size) {
size = offset;
mem[size] = '\0';
break;
}
}
fclose(f);
for (argc=1, p=mem+size-2; p >= mem; p--) {
argc += (*p == '\0');
}
argv = calloc(argc+1, sizeof(char *));
if (argv == NULL) {
perror("failed to allocate memory for argv");
free(mem);
exit(1);
}
for (p=mem, argc=0, offset=0; offset < size; argc++) {
argv[argc] = mem+offset;
offset += strlen(mem+offset)+1;
}
}
*argc_p = argc;
*argv_p = argv;
}
/*
* Linux x86 ABI requires the stack be 16 byte aligned. Keep things
* simple and just force it.
*/
#if defined(__i386__) || defined(__x86_64__)
#define __SO_FORCE_ARG_ALIGNMENT __attribute__((force_align_arg_pointer))
#else
#define __SO_FORCE_ARG_ALIGNMENT
#endif /* def some x86 */
/*
* Permit the compiler to override this one.
*/
#ifndef EXECABLE_INITIALIZE
#define EXECABLE_INITIALIZE do { } while(0)
#endif /* ndef EXECABLE_INITIALIZE */
/*
* Note, to avoid any runtime confusion, SO_MAIN is a void static
* function.
*/
#define SO_MAIN \
static void __execable_main(int, char**); \
__attribute__((visibility ("hidden"))) \
void __so_start(void); \
__SO_FORCE_ARG_ALIGNMENT \
void __so_start(void) \
{ \
int argc; \
char **argv; \
__execable_parse_args(&argc, &argv); \
EXECABLE_INITIALIZE; \
__execable_main(argc, argv); \
if (argc != 0) { \
free(argv[0]); \
free(argv); \
} \
exit(0); \
} \
static void __execable_main
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