Hi Am 19.04.23 um 20:30 schrieb Sui Jingfeng:
Hi, On 2023/4/19 23:46, Thomas Zimmermann wrote:Yeah, i915 carve out part of system ram as video memory, it is also called stolen memory.Hi Am 19.04.23 um 17:09 schrieb Daniel Vetter:On Tue, 18 Apr 2023 at 20:16, Sui Jingfeng <15330273260@xxxxxx> wrote:Hi, On 2023/4/19 01:52, Sui Jingfeng wrote:But honestly I do not have strong feel toward this, I just type what I'mHi, On 2023/4/18 16:32, Daniel Vetter wrote:On Mon, Apr 17, 2023 at 07:32:19PM +0800, Sui Jingfeng wrote:The fbdev test of IGT may write after EOF, which lead to out-of-bound access for the drm drivers using fbdev-generic. For example, on a x86So I read core some more and stumbled over drm_fb_helper_deferred_io().+ aspeed bmc card platform, with a 1680x1050 resolution display, runningfbdev test if IGT will cause the linux kernel hang with the followingcall trace: Oops: 0000 [#1] PREEMPT SMP PTI [IGT] fbdev: starting subtest eof Workqueue: events drm_fb_helper_damage_work [drm_kms_helper] [IGT] fbdev: starting subtest nullptr RIP: 0010:memcpy_erms+0xa/0x20 RSP: 0018:ffffa17d40167d98 EFLAGS: 00010246RAX: ffffa17d4eb7fa80 RBX: ffffa17d40e0aa80 RCX: 00000000000014c0 RDX: 0000000000001a40 RSI: ffffa17d40e0b000 RDI: ffffa17d4eb80000 RBP: ffffa17d40167e20 R08: 0000000000000000 R09: ffff89522ecff8c0 R10: ffffa17d4e4c5000 R11: 0000000000000000 R12: ffffa17d4eb7fa80 R13: 0000000000001a40 R14: 000000000000041a R15: ffffa17d40167e30FS: 0000000000000000(0000) GS:ffff895257380000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033CR2: ffffa17d40e0b000 CR3: 00000001eaeca006 CR4: 00000000001706e0Call Trace: <TASK>? drm_fbdev_generic_helper_fb_dirty+0x207/0x330 [drm_kms_helper]drm_fb_helper_damage_work+0x8f/0x170 [drm_kms_helper] process_one_work+0x21f/0x430 worker_thread+0x4e/0x3c0 ? __pfx_worker_thread+0x10/0x10 kthread+0xf4/0x120 ? __pfx_kthread+0x10/0x10 ret_from_fork+0x2c/0x50 </TASK> CR2: ffffa17d40e0b000 ---[ end trace 0000000000000000 ]--- The direct reason is that damage rectange computed by drm_fb_helper_memory_range_to_clip() does not guaranteed to be in-bound.It is already results in workaround code populate to elsewhere. Anotherreason is that exposing a larger buffer size than the actual needed helpto trigger this bug intrinsic in drm_fb_helper_memory_range_to_clip().Others fbdev emulation solutions write to the GEM buffer directly, they won't reproduce this bug because the .fb_dirty function callback do notbeing hooked, so no chance is given to drm_fb_helper_memory_range_to_clip() to generate a out-of-bound when drm_fb_helper_sys_write() is called. This patch break the trigger condition of this bug by shrinking the shadow buffer size to sizes->surface_height * buffer->fb->pitches[0]. Fixes: '8fbc9af55de0 ("drm/fbdev-generic: Set screen size to size of GEM buffer")' Signed-off-by: Sui Jingfeng <suijingfeng@xxxxxxxxxxx> --- drivers/gpu/drm/drm_fbdev_generic.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/drivers/gpu/drm/drm_fbdev_generic.c b/drivers/gpu/drm/drm_fbdev_generic.c index 8e5148bf40bb..b057cfbba938 100644 --- a/drivers/gpu/drm/drm_fbdev_generic.c +++ b/drivers/gpu/drm/drm_fbdev_generic.c @@ -94,7 +94,7 @@ static int drm_fbdev_generic_helper_fb_probe(struct drm_fb_helper *fb_helper, fb_helper->buffer = buffer; fb_helper->fb = buffer->fb; - screen_size = buffer->gem->size; + screen_size = sizes->surface_height * buffer->fb->pitches[0];Which has all the code and comments about this, including limiting. I think it would be clearer if we fix the issue there, instead of passing limits around in obscure places that then again get broken?No, it is more obscure doing that way... As the size of the shadow screen buffer will be exposed to userspace. The size 'helper->fb->height * helper->fb->pitches[0]' is a exactly(best) fit, You are guaranteed to waste at lease one byte by increasing one byte, and can not store all pixels by decreasing one byte (In the case where `helper->fb->pitches[0] = helper->fb->width * 4`). It implicitly tell the userspace do not go beyond that boundary. although userspace program can still choose to write after EOF, But it is for test purpose, to test the kernel if it can return a -EFBIG or not.The thing is,Thomas both authored the limit checks in drm_fb_helper_deferred_io() andthe patch which broken them again, so clearly this isn't very obvious. I'm thinking of something like this: diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c index ef4eb8b12766..726dab67c359 100644 --- a/drivers/gpu/drm/drm_fb_helper.c +++ b/drivers/gpu/drm/drm_fb_helper.c @@ -697,10 +697,7 @@ void drm_fb_helper_deferred_io(struct fb_info *info, struct list_head *pagerefli* of the screen and account for non-existing scanlines. Hence,* keep the covered memory area within the screen buffer. */ - if (info->screen_size) - total_size = info->screen_size; - else - total_size = info->fix.smem_len; + total_size = helper->fb->height * helper->fb->pitches[0];This is just to mitigate the mistakes already has been made, because it do not do a good splitting between the *clip* part and the *damage update* part. An ideal clipping do not obscure its updating backend with a out-of-bound damage rectangle. Why did the drm_fb_helper_memory_range_to_clip() can not do a good job in all case to pass its backend a always meaningful damage rect ?max_off = min(max_off, total_size); if (min_off < max_off) { I think that would make it utmost clear on what we're doing and why.Otherwise we're just going to re-create the same bug again, like we'vedone already :-)No, we create no bugs, we fix one. Thanks.understand without seeing you resend a V3. It's OK in overall, I will help to test this tomorrow. :-)Apologies for making you jump around all the time and doing different versions of the same bugfix :-/ I think this one here is ok to merge, I just thought when looking at the history that we revert the exact patch without any other changes or comments, and usually that means someone will come up with the same cleanup idea again, and then we'll have a bug again. So maybe a comment or a WARN_ON or something else would be good. I guess we could also do your patch, but put a WARN_ON that the computed total_size is never bigger than the drm_fb size into drm_fb_helper_deferred_io()? That would also make sure that this bug doesn't get resurrected again.We'd have to put this test into drm_fbdev_generic.c. Otherwise we'll break i915, which also uses deferred I/O, but without shadow buffering.. Maybe test in drm_fbdev_generic_helper_fb_dirty() if the clip rectangle extends the framebuffer size.I just learned it recently from i915 related document.But from what I'm understanding, It's still RAM in its nature, just reserved by firmware.Its bandwidth is extremely high, why not write to the GEM buffer directly? why deferred I/O pay off?
i915 doesn't use shadow buffering. I only flushes its caches in regular intervals: https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/i915/display/intel_fb.c#L1865 . The actual clipping rectangle is not relevant.
Best regards Thomas
Best regards Thomas-Daniel
-- Thomas Zimmermann Graphics Driver Developer SUSE Software Solutions Germany GmbH Frankenstrasse 146, 90461 Nuernberg, Germany GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman HRB 36809 (AG Nuernberg)
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