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Disable fixed framebuffer mmapping in virtual mode #10

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Disable fixed framebuffer mmapping in virtual mode #10

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22 changes: 21 additions & 1 deletion src/SDL/video_sdl.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -194,6 +194,12 @@ extern void SysMountFirstFloppy(void);
* Framebuffer allocation routines
*/

/* Fixed mmap defeats normal precautions about not overlapping your program memory.
* These using fixed mmap shenanigans always are a hair's breadth away from just overlapping another malloc
* and causing severe memory corruption. We can't just release the framebuffer for a length of time and
* expect things not to go sideways.
*/

static void *vm_acquire_framebuffer(uint32 size)
{
// always try to reallocate framebuffer at the same address
Expand Down Expand Up @@ -707,8 +713,16 @@ void driver_base::init()
if (!use_vosf) {
// Allocate memory for frame buffer
the_buffer_size = (aligned_height + 2) * s->pitch;
the_buffer_copy = (uint8 *)calloc(1, the_buffer_size);
// due to mmap MAP_FIXED shenanigans it is extremely important that vm_acquire_framebuffer
// happens before other allocations
#if DIRECT_ADDRESSING || REAL_ADDRESSING
the_buffer = (uint8 *)vm_acquire_framebuffer(the_buffer_size);
#else
the_buffer = (uint8 *)calloc(1, the_buffer_size);
if (the_buffer == NULL)
the_buffer = (uint8 *)VM_MAP_FAILED;
#endif
the_buffer_copy = (uint8 *)calloc(1, the_buffer_size);
D(bug("the_buffer = %p, the_buffer_copy = %p\n", the_buffer, the_buffer_copy));
}

Expand Down Expand Up @@ -767,12 +781,18 @@ driver_base::~driver_base()
if (s)
SDL_FreeSurface(s);

#if DIRECT_ADDRESSING || REAL_ADDRESSING
// the_buffer shall always be mapped through vm_acquire_framebuffer()
if (the_buffer != VM_MAP_FAILED) {
D(bug(" releasing the_buffer at %p (%d bytes)\n", the_buffer, the_buffer_size));
vm_release_framebuffer(the_buffer, the_buffer_size);
the_buffer = NULL;
}
#else
if (the_buffer != VM_MAP_FAILED) {
free(the_buffer);
}
#endif

// Free frame buffer(s)
if (!use_vosf) {
Expand Down