From df67b3daea602728b51325a4debaeeb912ee51d1 Mon Sep 17 00:00:00 2001 From: Jason Baron Date: Fri, 29 Sep 2006 01:58:58 -0700 Subject: [PATCH] make PROT_WRITE imply PROT_READ Make PROT_WRITE imply PROT_READ for a number of architectures which don't support write only in hardware. While looking at this, I noticed that some architectures which do not support write only mappings already take the exact same approach. For example, in arch/alpha/mm/fault.c: " if (cause < 0) { if (!(vma->vm_flags & VM_EXEC)) goto bad_area; } else if (!cause) { /* Allow reads even for write-only mappings */ if (!(vma->vm_flags & (VM_READ | VM_WRITE))) goto bad_area; } else { if (!(vma->vm_flags & VM_WRITE)) goto bad_area; } " Thus, this patch brings other architectures which do not support write only mappings in-line and consistent with the rest. I've verified the patch on ia64, x86_64 and x86. Additional discussion: Several architectures, including x86, can not support write-only mappings. The pte for x86 reserves a single bit for protection and its two states are read only or read/write. Thus, write only is not supported in h/w. Currently, if i 'mmap' a page write-only, the first read attempt on that page creates a page fault and will SEGV. That check is enforced in arch/blah/mm/fault.c. However, if i first write that page it will fault in and the pte will be set to read/write. Thus, any subsequent reads to the page will succeed. It is this inconsistency in behavior that this patch is attempting to address. Furthermore, if the page is swapped out, and then brought back the first read will also cause a SEGV. Thus, any arbitrary read on a page can potentially result in a SEGV. According to the SuSv3 spec, "if the application requests only PROT_WRITE, the implementation may also allow read access." Also as mentioned, some archtectures, such as alpha, shown above already take the approach that i am suggesting. The counter-argument to this raised by Arjan, is that the kernel is enforcing the write only mapping the best it can given the h/w limitations. This is true, however Alan Cox, and myself would argue that the inconsitency in behavior, that is applications can sometimes work/sometimes fails is highly undesireable. If you read through the thread, i think people, came to an agreement on the last patch i posted, as nobody has objected to it... Signed-off-by: Jason Baron Cc: Russell King Cc: "Luck, Tony" Cc: Hugh Dickins Cc: Roman Zippel Cc: Geert Uytterhoeven Cc: Paul Mackerras Cc: Benjamin Herrenschmidt Acked-by: Andi Kleen Acked-by: Alan Cox Cc: Arjan van de Ven Acked-by: Paul Mundt Cc: Kazumoto Kojima Cc: Ian Molton Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- arch/sh/mm/fault.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'arch/sh/mm') diff --git a/arch/sh/mm/fault.c b/arch/sh/mm/fault.c index c69fd603226..507f2891470 100644 --- a/arch/sh/mm/fault.c +++ b/arch/sh/mm/fault.c @@ -69,7 +69,7 @@ good_area: if (!(vma->vm_flags & VM_WRITE)) goto bad_area; } else { - if (!(vma->vm_flags & (VM_READ | VM_EXEC))) + if (!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))) goto bad_area; } -- cgit v1.2.3