From fa3c109a6d302b56437a6412c5f3044c3e12de03 Mon Sep 17 00:00:00 2001 From: Yosry Ahmed Date: Sat, 10 Aug 2024 00:59:11 +0000 Subject: [PATCH] dma-mapping: use bit masking to check VM_DMA_COHERENT In dma_common_find_pages(), area->flags are compared directly with VM_DMA_COHERENT. This works because VM_DMA_COHERENT is the only set flag. During development of a new feature (ASI [1]), a new VM flag is introduced, and that flag can be injected into VM_DMA_COHERENT mappings (among others). The presence of that flag caused dma_common_find_pages() to return NULL for VM_DMA_COHERENT addresses, leading to a lot of problems ending in crashing during boot. It took a bit of time to figure this problem out. It was a mistake to inject a VM flag to begin with, but it took a significant amount of debugging to figure out the problem. Most users of area->flags use bitmasking rather than equivalency to check for flags. Update dma_common_find_pages() and dma_common_free_remap() to do the same, which would have avoided the boot crashing. Instead, add a warning in dma_common_find_pages() if any extra VM flags are set to catch such problems more easily during development. No functional change intended. [1]https://lore.kernel.org/lkml/20240712-asi-rfc-24-v1-0-144b319a40d8@google.com/ Signed-off-by: Yosry Ahmed Signed-off-by: Christoph Hellwig --- kernel/dma/remap.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/kernel/dma/remap.c b/kernel/dma/remap.c index 27596f3b4aef..9e2afad1c615 100644 --- a/kernel/dma/remap.c +++ b/kernel/dma/remap.c @@ -10,8 +10,10 @@ struct page **dma_common_find_pages(void *cpu_addr) { struct vm_struct *area = find_vm_area(cpu_addr); - if (!area || area->flags != VM_DMA_COHERENT) + if (!area || !(area->flags & VM_DMA_COHERENT)) return NULL; + WARN(area->flags != VM_DMA_COHERENT, + "unexpected flags in area: %p\n", cpu_addr); return area->pages; } @@ -61,7 +63,7 @@ void dma_common_free_remap(void *cpu_addr, size_t size) { struct vm_struct *area = find_vm_area(cpu_addr); - if (!area || area->flags != VM_DMA_COHERENT) { + if (!area || !(area->flags & VM_DMA_COHERENT)) { WARN(1, "trying to free invalid coherent area: %p\n", cpu_addr); return; }