/* Heavily arch-specific, I'm afraid.. */
#if defined(i386_TARGET_ARCH)
+
+/* Now here's something obscure for you:
+
+ When generating an adjustor thunk that uses the C calling
+ convention, we have to make sure that the thunk kicks off
+ the process of jumping into Haskell with a tail jump. Why?
+ Because as a result of jumping in into Haskell we may end
+ up freeing the very adjustor thunk we came from using
+ freeHaskellFunctionPtr(). Hence, we better not return to
+ the adjustor code on our way out, since it could by then
+ point to junk.
+
+ The fix is readily at hand, just include the opcodes
+ for the C stack fixup code that we need to perform when
+ returning in some static piece of memory and arrange
+ to return to it before tail jumping from the adjustor thunk.
+
+ For this to work we make the assumption that bytes in .data
+ are considered executable.
+*/
+static unsigned char __obscure_ccall_ret_code [] =
+ { 0x83, 0xc4, 0x04 /* addl $0x4, %esp */
+ , 0xc3 /* ret */
+ };
+
void*
createAdjustor(int cconv, StgStablePtr hptr, StgFunPtr wptr)
{
void *adjustor;
unsigned char* adj_code;
size_t sizeof_adjustor;
-
+
if (cconv == 0) { /* the adjustor will be _stdcall'ed */
/* Magic constant computed by inspecting the code length of
the following assembly language snippet
(offset and machine code prefixed):
- <00>: 68 ef be ad de pushl $0xdeadbeef # constant is large enough to
+ <00>: 68 ef be ad de pushl $0xdeadbeef # constant is large enough to
# hold a StgStablePtr
- <05>: b8 fa ef ff 00 movl $0x00ffeffa, %eax # load up wptr
- <0a>: ff d0 call %eax # and call it.
- <0c>: 83 c4 04 addl $0x4,%esp # remove stable pointer.
- <0f>: c3 ret # return to where you came from.
+ <05>: b8 fa ef ff 00 movl $0x00ffeffa, %eax # load up wptr
+ <0a>: 68 ef be ad de pushl $__obscure_ccall_ret_code # push the return address
+ <0f>: ff e0 jmp *%eax # jump to wptr
The ccall'ing version is a tad different, passing in the return
address of the caller to the auto-generated C stub (which enters
via the stable pointer.) (The auto-generated C stub is in on this
game, don't worry :-)
- The adjustor makes the assumption that any return value
+ See the comment next to __obscure_ccall_ret_code why we need to
+ perform a tail jump instead of a call, followed by some C stack
+ fixup.
+
+ Note: The adjustor makes the assumption that any return value
coming back from the C stub is not stored on the stack.
That's (thankfully) the case here with the restricted set of
return types that we support.
+
+
*/
- sizeof_adjustor = 16*sizeof(char);
+ sizeof_adjustor = 17*sizeof(char);
if ((adjustor = stgMallocBytes(sizeof_adjustor,"createAdjustor")) == NULL) {
return NULL;
adj_code[0x05] = (unsigned char)0xb8; /* movl $wptr, %eax */
*((StgFunPtr*)(adj_code + 0x06)) = (StgFunPtr)wptr;
- adj_code[0x0a] = (unsigned char)0xff; /* call %eax */
- adj_code[0x0b] = (unsigned char)0xd0;
-
- adj_code[0x0c] = (unsigned char)0x83; /* addl $0x4, %esp */
- adj_code[0x0d] = (unsigned char)0xc4;
- adj_code[0x0e] = (unsigned char)0x04;
-
- adj_code[0x0f] = (unsigned char)0xc3; /* ret */
+ adj_code[0x0a] = (unsigned char)0x68; /* pushl __obscure_ccall_ret_code */
+ *((StgFunPtr*)(adj_code + 0x0b)) = (StgFunPtr)__obscure_ccall_ret_code;
+ adj_code[0x0f] = (unsigned char)0xff; /* jmp *%eax */
+ adj_code[0x10] = (unsigned char)0xe0;
+
}
/* Have fun! */