Re: calling select, via syscall, on 64 bit Leopard.

Subject: Re: calling select, via syscall, on 64 bit Leopard.
From: Jean-Daniel Dupas <email@hidden>
Date: Wed, 3 Sep 2008 12:11:57 +0200

Le 3 sept. 08 à 11:48, Luke Daley a écrit :

Hi,

This was the select and svn discussion, but we have narrowed the problem down to syscall() on 64 bit Leopard so I am starting a new thread.

`man syscall` tells us that you need to use __syscall() on 64 bit architectures. The problem with that though is that __syscall() is in libSystem, but is not exported. Jean-Daniel Dupas then contributed this code to get to __syscall()…

#include <libc.h>
#include <mach-o/nlist.h>

typedef int (*__syscall_t)(quad_t number, ...);

int main(int argc, const char * argv[]) {
struct nlist nl[1];
bzero(&nl, sizeof(struct nlist) * 2);
nl[0].n_un.n_name = (char *)"___syscall";
if (nlist("/usr/lib/libSystem.dylib", nl) < 0 || nl[0].n_type == N_UNDF) {
   fprintf(stderr, "nlist(%s, %s) failed\n",
           "/usr/lib/libSystem.dylib",
           nl[0].n_un.n_name);
   return -1;
}
__syscall_t fcn = (__syscall_t)nl[0].n_value;

return 0;
}

Except that gives "error: ‘union <anonymous>’ has no member named ‘n_name’".

Looking at mach-o/nlist.h…

struct nlist {
union {
#ifndef __LP64__
char *n_name; /* for use when in-core */
#endif
int32_t n_strx; /* index into the string table */
} n_un;
uint8_t n_type; /* type flag, see below */
uint8_t n_sect; /* section number or NO_SECT */
int16_t n_desc; /* see <mach-o/stab.h> */
uint32_t n_value; /* value of this symbol (or stab offset) */
};

That looks to me line n_name isn't there on 64 bit architectures.

Can I get at this any other way?

--

I just have a question. Why do you need syscall ? How are you currently performing your interposition ?

I think using the following technic, you don't have to use syscall at all. Create a dylib that contains your interposition code, and then use the DYLD_INSERT_LIBRARIES env to load your dylib into the target application.

* interpose.c

* static

* int

* my_open(const char* path, int flags, mode_t mode)

* {

* int value;

* // do stuff before open (including changing the arguments)

* value = open(path, flags, mode);

* // do stuff after open (including changing the return value(s))

* return value;

* }

* DYLD_INTERPOSE(my_open, open)

#define DYLD_INTERPOSE(_replacment,_replacee) \

__attribute__((used)) static struct{ const void* replacment; const void* replacee; } _interpose_##_replacee \

__attribute__ ((section ("__DATA,__interpose"))) = { (const void*)(unsigned long)&_replacment, (const void*)(unsigned long)&_replacee };

int my_open(const char *, int, mode_t);

int my_close(int);

DYLD_INTERPOSE(my_open, open);

DYLD_INTERPOSE(my_close, close);

int my_open(const char *path, int flags, mode_t mode) {

int ret = open(path, flags, mode);

printf("--> %d = open(%s, %x, %x)\n", ret, path, flags, mode);

return ret;

}

int my_close(int d) {

int ret = close(d);

printf("--> %d = close(%d)\n", ret, d);

return ret;

}

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