ports/120052: SDL macro on x86 violates GCC and SYS V ABI assumptions

[Christoph Mallon]

>Number:         120052
>Category:       ports
>Synopsis:       SDL macro on x86 violates GCC and SYS V ABI assumptions
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    freebsd-ports-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Sun Jan 27 17:20:01 UTC 2008
>Closed-Date:
>Last-Modified:
>Originator:     Christoph Mallon
>Release:        n/a
>Organization:
-
>Environment:
n/a
>Description:
The x86 inline assembler version of the SDL macro SDL_revcpy() sets the direction flag ("std"), but does not clear it ("cld").  This is invalid according to GCC (inline assembler, which sets the direction flag, must reset it[0]) and the SYS V ABI (functions must leave with the direction flag cleared[1]). The macro is (indirectly, exact call sequence below) used in SDL_BlitSurface(), so this call sometimes returns with the direction flag set. This happens for bliting a surface onto itself with the destination coordinates set right/down of the source coordinates (typical use of this is scrolling left/up). Later on other parts of the code (like inlined memcpy()) cause memory corruption.

Call sequence:
SDL_BlitSurface() (#define SDL_Blit_Surface SDL_BlitUpper)
SDL_BlitUpper()
SDL_BlitLower()
src->map->sw_blit() (function pointer to SDL_SoftBlit())
SDL_SoftBlit()
src->map->sw_data->blit() (function pointer to SDL_BlitCopyOverlap())
SDL_BlitCopyOverlap()
SDL_revcpy()


[0] http://gcc.gnu.org/gcc-4.3/changes.html - "IA-32/x86-64", fourth bullet
[1] http://www.sco.com/developers/devspecs/abi386-4.pdf - page 38 EFLAGS
>How-To-Repeat:
This program demonstrates the problem (compile with "cc -O `sdl-config --cflags --libs` $FILENAME"):

#include <SDL.h>
#include <stdio.h>
#include <stdlib.h>

static inline int TestDirectionFlag(void)
{
	unsigned eflags;
	__asm__ __volatile__("pushf\n\tpop %0" : "=r" (eflags));
	return (eflags & 0x400) != 0;
}

int main(void)
{
	SDL_Init(SDL_INIT_VIDEO);
	SDL_Surface* const s = SDL_SetVideoMode(640, 480, 16, SDL_SWSURFACE);

	SDL_Rect src_rect = {  0,  0, 630, 470 };
	SDL_Rect dst_rect = { 10, 10, 630, 470 };
	if (TestDirectionFlag())
	{
		fputs("direction flag set BEFORE SDL_BlitSurface()\n", stderr);
		abort();
	}
	SDL_BlitSurface(s, &src_rect, s, &dst_rect);
	if (TestDirectionFlag())
	{
		fputs("direction flag set AFTER SDL_BlitSurface()\n", stderr);
		abort();
	}

	SDL_Quit();
	return 0;
}
>Fix:
Add a "cld" in the macro SDL_revcpy(), see attached patch.

Patch attached with submission follows:

--- include/SDL_stdinc.h.orig	2006-06-27 06:48:32.000000000 +0200
+++ include/SDL_stdinc.h	2008-01-27 16:39:16.000000000 +0100
@@ -319,6 +319,7 @@
 	__asm__ __volatile__ (				\
 		"std\n\t"				\
 		"rep ; movsl\n\t"			\
+		"cld\n\t"				\
 		: "=&c" (u0), "=&D" (u1), "=&S" (u2)	\
 		: "0" (n >> 2),				\
 		  "1" (dstp+(n-4)), "2" (srcp+(n-4))	\


>Release-Note:
>Audit-Trail:
>Unformatted: