Bug#462794: Direction flag not cleared

[Kai Wb.]

Package: libsdl1.2-dev
Version: 1.2.13-1
Severity: important
Justification: Violation of SYS V ABI specification

Hello,
Christoph Mallon, the driving coder behind ja2 stracciatella [0], has discovered
a serious problem. libsdl is violating the SYS V ABI (x86) [1]. There it is
stated that the "direction flag must be set to the "forward" (that is, zero)
direction before entry and upon exit." [2]. But as the program in the attachment
demonstrates ([3]) libsdl doesn't do that always.

If you use SDL_BlitSurface(), the direction flag is set after exiting the
function. This problem occurs because SDL_BlitSurface() calls SDL_revcpy(), the
sequence is:
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()

SDL_revcpy() sets but doesn't clear the direction flag. The incriminating code
is to be found in SDL_stdinc.h:
> #if defined(__GNUC__) && defined(i386)
> #define SDL_revcpy(dst, src, len)			\
> do {							\
> 	int u0, u1, u2;					\
> 	char *dstp = (char *)(dst);			\
> 	char *srcp = (char *)(src);			\
> 	int n = (len);					\
> 	if ( n >= 4 ) {					\
> 	__asm__ __volatile__ (				\
> 		"std\n\t"				\
> 		"rep ; movsl\n\t"			\
> 		: "=&c" (u0), "=&D" (u1), "=&S" (u2)	\
> 		: "0" (n >> 2),				\
> 		  "1" (dstp+(n-4)), "2" (srcp+(n-4))	\
> 		: "memory" );				\
> 	}						\
> 	switch (n & 3) {				\
> 		case 3: dstp[2] = srcp[2];		\
> 		case 2: dstp[1] = srcp[1];		\
> 		case 1: dstp[0] = srcp[0];		\
> 			break;				\
> 		default:				\
> 			break;				\
> 	}						\
> } while(0)
> #endif

As a sidenote: in the GCC 4.3 changelog it is stated that: "It is now invalid to
set the flag in asm statement without reseting it afterward.".

Kind regards,
Kai

P.S.: Please CC Christoph always!

[0] http://ja2.dragonriders.de/
[1] http://www.sco.com/developers/devspecs/abi386-4.pdf
[2] [1], page 38, EFLAGS
[3] Used build command: cc -O `sdl-config --cflags --libs` sdl_bug.c


-- System Information:
Debian Release: lenny/sid
  APT prefers testing
  APT policy: (500, 'testing')
Architecture: i386 (i686)

Kernel: Linux 2.6.22.10-cabal
Locale: LANG=de_DE.UTF-8, LC_CTYPE=de_DE.UTF-8 (charmap=UTF-8) (ignored: LC_ALL
set to de_DE.UTF-8)
Shell: /bin/sh linked to /bin/bash



-- 

Kai Wasserbäch (Kai Wasserbaech)

E-Mail: debian at carbon-project.org
Jabber (debianforum.de): Drizzt
URL: http://wiki.debianforum.de/Drizzt_Do%27Urden
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