| Message ID | 1429789576-19570-1-git-send-email-m.szyprowski@samsung.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Thu, Apr 23, 2015 at 12:46:16PM +0100, Marek Szyprowski wrote: > Buffers allocated by dma_alloc_coherent() are always zeroed on Alpha, > ARM (32bit), MIPS, PowerPC, x86/x86_64 and probably other architectures. That's pretty compelling, even if it sucks. Maybe we could add a debug option to poison DMA buffers if __GFP_ZERO isn't passed? It might help others avoid the painful debugging experience you had... Will
diff --git a/arch/arm64/mm/dma-mapping.c b/arch/arm64/mm/dma-mapping.c index ef7d112..e0f14ee 100644 --- a/arch/arm64/mm/dma-mapping.c +++ b/arch/arm64/mm/dma-mapping.c @@ -67,8 +67,7 @@ static void *__alloc_from_pool(size_t size, struct page **ret_page, gfp_t flags) *ret_page = phys_to_page(phys); ptr = (void *)val; - if (flags & __GFP_ZERO) - memset(ptr, 0, size); + memset(ptr, 0, size); } return ptr; @@ -113,8 +112,7 @@ static void *__dma_alloc_coherent(struct device *dev, size_t size, *dma_handle = phys_to_dma(dev, page_to_phys(page)); addr = page_address(page); - if (flags & __GFP_ZERO) - memset(addr, 0, size); + memset(addr, 0, size); return addr; } else { return swiotlb_alloc_coherent(dev, size, dma_handle, flags);
Buffers allocated by dma_alloc_coherent() are always zeroed on Alpha, ARM (32bit), MIPS, PowerPC, x86/x86_64 and probably other architectures. It turned out that some drivers rely on this 'feature'. Allocated buffer might be also exposed to userspace with dma_mmap() call, so clearing it is desired from security point of view to avoid exposing random memory to userspace. This patch unifies dma_alloc_coherent() behavior on ARM64 architecture with other implementations by unconditionally zeroing allocated buffer. Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com> --- Hello, This issue was really nasty to debug. The problem has been observed with dw_mmc driver on Exynos 5433. I've also check if there are any calls to dma_alloc_coherent() with __GFP_ZERO flag and found only two drivers in the whole kernel tree: drivers/staging/android/ion/ion_cma_heap.c and drivers/thunderbolt/nhi.c. The best will be to review all drivers and add missing __GFP_ZERO flag or replace it with dma_zalloc_coherent() and only then convert all implementation for all architectures to honor this flag. However I expect that this is a huge task and for now ARM64 should just match the behavior of other architectures. Best regards Marek Szyprowski Samsung Poland R&D Center --- arch/arm64/mm/dma-mapping.c | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-)