Vegard: /* Definition */ fix ridiculous typo

2020-02-14T18:32:24Z

Definition: fix ridiculous typo

← Older revision		Revision as of 18:32, 14 February 2020
Line 10:		Line 10:

	#ifndef PAGE_SIZE		#ifndef PAGE_SIZE
	#define PAGE_SIZE ~~4095~~		#define PAGE_SIZE 4096
	#endif		#endif

Vegard: /* Definition */

2020-01-31T08:47:48Z

Definition

← Older revision		Revision as of 08:47, 31 January 2020
Line 53:		Line 53:
	}		}

	void free_thunk(~~thinked_fn~~ fn)		void free_thunk(thunked_fn fn)
	{		{
	munmap((void *) fn, PAGE_SIZE);		munmap((void *) fn, PAGE_SIZE);

Vegard: new page

2020-01-31T07:45:26Z

new page

New page

It is possible in C to "bake" one or more arguments into a function pointer that takes no arguments, thus solving the problem of libraries that take a callback function without a data pointer.

Below is one possible implementation for x86-64 that bakes a single argument into a thunk. Error handling may need to be adjusted for specific use cases.

=== Definition ===

<source lang="C">
#include <sys/mman.h>
#include <stdlib.h>

#ifndef PAGE_SIZE
#define PAGE_SIZE 4095
#endif

typedef void (*unthunked_fn)(void *);
typedef void (*thunked_fn)(void);

thunked_fn make_thunk(unthunked_fn fn, void *data)
{
void *ptr = mmap(NULL, PAGE_SIZE,
PROT_READ | PROT_WRITE | PROT_EXEC,
MAP_PRIVATE | MAP_ANONYMOUS,
-1, 0);
if (ptr == MAP_FAILED)
return NULL;

struct {
unsigned char movabs_rdi[10];
unsigned char movabs_rax[10];
unsigned char jmpq[2];
} *page = ptr;

/* movabs $..., %rdi */
page->movabs_rdi[0] = 0x48;
page->movabs_rdi[1] = 0xbf;
for (unsigned int i = 0; i < 8; ++i)
page->movabs_rdi[2 + i] = (unsigned long) data >> (8 * i);

/* movabs $..., %rax */
page->movabs_rax[0] = 0x48;
page->movabs_rax[1] = 0xb8;
for (unsigned int i = 0; i < 8; ++i)
page->movabs_rax[2 + i] = (unsigned long) fn >> (8 * i);

/* jmpq %rax */
page->jmpq[0] = 0xff;
page->jmpq[1] = 0xe0;

/* Optional; only needed for CPUs with W^X */
mprotect(ptr, PAGE_SIZE, PROT_READ | PROT_EXEC);

return (thunked_fn) page;
}

void free_thunk(thinked_fn fn)
{
munmap((void *) fn, PAGE_SIZE);
}
</source>

=== Usage ===

<source lang="C">
#include <stdio.h>

static void print_str(void *data)
{
printf("print_str: %s\n", (const char *) data);
}

int main(int argc, char *argv[])
{
thunked_fn fn = make_thunk(print_str, "Hello world!");

/* Prints "Hello world!" */
fn();

return 0;
}
</source>

== See also ==

* "C Closures as a Library": https://nullprogram.com/blog/2017/01/08/

[[Category:Programming]]
[[Category:C]]

Thunk - Revision history

Vegard: /* Definition */ fix ridiculous typo

Vegard: /* Definition */

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