Re: [PATCH 1/3] drm/panfrost: Simplify lock_region calculation
On 23 August 2021 22:09:08 BST, Alyssa Rosenzweig wrote: >> > In lock_region, simplify the calculation of the region_width parameter. >> > This field is the size, but encoded as log2(ceil(size)) - 1. >> > log2(ceil(size)) may be computed directly as fls(size - 1). However, we >> > want to use the 64-bit versions as the amount to lock can exceed >> > 32-bits. >> > >> > This avoids undefined behaviour when locking all memory (size ~0), >> > caught by UBSAN. >> >> It might have been useful to mention what it is that UBSAN specifically >> picked up (it took me a while to spot) - but anyway I think there's a >> bigger issue with it being completely wrong when size == ~0 (see below). > >Indeed. I've updated the commit message in v2 to explain what goes >wrong (your analysis was spot on, but a mailing list message is more >ephermal than a commit message). I'll send out v2 tomorrow assuming >nobody objects to v1 in the mean time. > >Thanks for the review. > >> There is potentially a third bug which kbase only recently attempted to >> fix. The lock address is effectively rounded down by the hardware (the >> bottom bits are ignored). So if you have mask=(1<> (iova & mask) != ((iova + size) & mask) then you are potentially failing >> to lock the end of the intended region. kbase has added some code to >> handle this: >> >> >/* Round up if some memory pages spill into the next region. */ >> >region_frame_number_start = pfn >> (lockaddr_size_log2 - PAGE_SHIFT); >> >region_frame_number_end = >> >(pfn + num_pages - 1) >> (lockaddr_size_log2 - PAGE_SHIFT); >> > >> >if (region_frame_number_start < region_frame_number_end) >> >lockaddr_size_log2 += 1; >> >> I guess we should too? > >Oh, I missed this one. Guess we have 4 bugs with this code instead of >just 3, yikes. How could such a short function be so deeply and horribly >broken? > >Should I add a fourth patch to the series to fix this? Yes please! Thanks, Steve
Re: [PATCH 1/3] drm/panfrost: Simplify lock_region calculation
> > In lock_region, simplify the calculation of the region_width parameter. > > This field is the size, but encoded as log2(ceil(size)) - 1. > > log2(ceil(size)) may be computed directly as fls(size - 1). However, we > > want to use the 64-bit versions as the amount to lock can exceed > > 32-bits. > > > > This avoids undefined behaviour when locking all memory (size ~0), > > caught by UBSAN. > > It might have been useful to mention what it is that UBSAN specifically > picked up (it took me a while to spot) - but anyway I think there's a > bigger issue with it being completely wrong when size == ~0 (see below). Indeed. I've updated the commit message in v2 to explain what goes wrong (your analysis was spot on, but a mailing list message is more ephermal than a commit message). I'll send out v2 tomorrow assuming nobody objects to v1 in the mean time. Thanks for the review. > There is potentially a third bug which kbase only recently attempted to > fix. The lock address is effectively rounded down by the hardware (the > bottom bits are ignored). So if you have mask=(1< (iova & mask) != ((iova + size) & mask) then you are potentially failing > to lock the end of the intended region. kbase has added some code to > handle this: > > > /* Round up if some memory pages spill into the next region. */ > > region_frame_number_start = pfn >> (lockaddr_size_log2 - PAGE_SHIFT); > > region_frame_number_end = > > (pfn + num_pages - 1) >> (lockaddr_size_log2 - PAGE_SHIFT); > > > > if (region_frame_number_start < region_frame_number_end) > > lockaddr_size_log2 += 1; > > I guess we should too? Oh, I missed this one. Guess we have 4 bugs with this code instead of just 3, yikes. How could such a short function be so deeply and horribly broken? Should I add a fourth patch to the series to fix this?
Re: [PATCH 1/3] drm/panfrost: Simplify lock_region calculation
On 20/08/2021 22:31, Alyssa Rosenzweig wrote:
> In lock_region, simplify the calculation of the region_width parameter.
> This field is the size, but encoded as log2(ceil(size)) - 1.
> log2(ceil(size)) may be computed directly as fls(size - 1). However, we
> want to use the 64-bit versions as the amount to lock can exceed
> 32-bits.
>
> This avoids undefined behaviour when locking all memory (size ~0),
> caught by UBSAN.
It might have been useful to mention what it is that UBSAN specifically
picked up (it took me a while to spot) - but anyway I think there's a
bigger issue with it being completely wrong when size == ~0 (see below).
> Signed-off-by: Alyssa Rosenzweig
> Reported-and-tested-by: Chris Morgan
> Cc:
However, I've confirmed this returns the same values and is certainly
more simple, so:
Reviewed-by: Steven Price
> ---
> drivers/gpu/drm/panfrost/panfrost_mmu.c | 19 +--
> 1 file changed, 5 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/gpu/drm/panfrost/panfrost_mmu.c
> b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> index 0da5b3100ab1..f6e02d0392f4 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_mmu.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> @@ -62,21 +62,12 @@ static void lock_region(struct panfrost_device *pfdev,
> u32 as_nr,
> {
> u8 region_width;
> u64 region = iova & PAGE_MASK;
> - /*
> - * fls returns:
> - * 1 .. 32
> - *
> - * 10 + fls(num_pages)
> - * results in the range (11 .. 42)
> - */
> -
> - size = round_up(size, PAGE_SIZE);
This seems to be the first issue - ~0 will be 'rounded up' to 0.
>
> - region_width = 10 + fls(size >> PAGE_SHIFT);
fls(0) == 0, so region_width == 10.
> - if ((size >> PAGE_SHIFT) != (1ul << (region_width - 11))) {
Presumably here's where UBSAN objects - we're shifting by a negative
value, which even it it happens to works means the lock region is tiny
and certainly not what was intended! It might well be worth a:
Fixes: f3ba91228e8e ("drm/panfrost: Add initial panfrost driver")
Note for anyone following along at (working-from-) home: although this
code was cargo culted from kbase - kbase is fine because it takes a pfn
and doesn't do the round_up() stage.
Which also exposes the second bug (fixed in patch 2): a size_t isn't big
enough on 32 bit platforms (all Midgard/Bifrost GPUs have a VA size
bigger than 32 bits). Again kbase gets away with a u32 because it's a pfn.
There is potentially a third bug which kbase only recently attempted to
fix. The lock address is effectively rounded down by the hardware (the
bottom bits are ignored). So if you have mask=(1< /* Round up if some memory pages spill into the next region. */
> region_frame_number_start = pfn >> (lockaddr_size_log2 - PAGE_SHIFT);
> region_frame_number_end =
> (pfn + num_pages - 1) >> (lockaddr_size_log2 - PAGE_SHIFT);
>
> if (region_frame_number_start < region_frame_number_end)
> lockaddr_size_log2 += 1;
I guess we should too?
Steve
> - /* not pow2, so must go up to the next pow2 */
> - region_width += 1;
> - }
> + /* The size is encoded as ceil(log2) minus(1), which may be calculated
> + * with fls. The size must be clamped to hardware bounds.
> + */
> + size = max_t(u64, size, PAGE_SIZE);
> + region_width = fls64(size - 1) - 1;
> region |= region_width;
>
> /* Lock the region that needs to be updated */
>
[PATCH 1/3] drm/panfrost: Simplify lock_region calculation
In lock_region, simplify the calculation of the region_width parameter.
This field is the size, but encoded as log2(ceil(size)) - 1.
log2(ceil(size)) may be computed directly as fls(size - 1). However, we
want to use the 64-bit versions as the amount to lock can exceed
32-bits.
This avoids undefined behaviour when locking all memory (size ~0),
caught by UBSAN.
Signed-off-by: Alyssa Rosenzweig
Reported-and-tested-by: Chris Morgan
Cc:
---
drivers/gpu/drm/panfrost/panfrost_mmu.c | 19 +--
1 file changed, 5 insertions(+), 14 deletions(-)
diff --git a/drivers/gpu/drm/panfrost/panfrost_mmu.c
b/drivers/gpu/drm/panfrost/panfrost_mmu.c
index 0da5b3100ab1..f6e02d0392f4 100644
--- a/drivers/gpu/drm/panfrost/panfrost_mmu.c
+++ b/drivers/gpu/drm/panfrost/panfrost_mmu.c
@@ -62,21 +62,12 @@ static void lock_region(struct panfrost_device *pfdev, u32
as_nr,
{
u8 region_width;
u64 region = iova & PAGE_MASK;
- /*
-* fls returns:
-* 1 .. 32
-*
-* 10 + fls(num_pages)
-* results in the range (11 .. 42)
-*/
-
- size = round_up(size, PAGE_SIZE);
- region_width = 10 + fls(size >> PAGE_SHIFT);
- if ((size >> PAGE_SHIFT) != (1ul << (region_width - 11))) {
- /* not pow2, so must go up to the next pow2 */
- region_width += 1;
- }
+ /* The size is encoded as ceil(log2) minus(1), which may be calculated
+* with fls. The size must be clamped to hardware bounds.
+*/
+ size = max_t(u64, size, PAGE_SIZE);
+ region_width = fls64(size - 1) - 1;
region |= region_width;
/* Lock the region that needs to be updated */
--
2.30.2
