On Mon, Jul 30, 2012 at 12:28:35AM +0200, Sasha Levin wrote:
> Hi all,
>
> I was poking around /dev/kmem related code, and noticed the following in
> mmap_kmem():
>
> /* Turn a kernel-virtual address into a physical page frame */
> pfn = __pa((u64)vma->vm_pgoff << PAGE_SHIFT) >> PAGE_SHIFT;
>
> Which looked odd since vm_pgoff is the offset into the mapping, so
> I'd assume that PAGE_OFFSET should be added to it as well, otherwise
> we get an invalid address.
It's supposed to be used with kernel offsets in the first place,
i.e. vma->vm_pgoff << PAGE_SHIFT should actually be a kernel virtual
address. See 6d3154c Revert "[PATCH] Fix up mmap_kmem".
> I tested it by writing something like this:
>
> int main(void)
> {
> int fd;
> void *addr;
>
> fd = open("/dev/kmem", O_RDONLY);
> addr = mmap(NULL, 4096, PROT_READ, MAP_PRIVATE, fd, 4096);
>
> return 0;
> }
>
> Which indeed triggered a VM_BUG:
>
> [ 32.285431] kernel BUG at arch/x86/mm/physaddr.c:18!
x86's debug-version of __pa() triggers that bug. I'm reluctant to add
a whole lot of error checking to this interface, given that you should
already know what you are doing. OTOH, crashing like this is not very
nice, either.
Is there a portable way to check if an address is a kernel virtual
one? It looks like comparing to PAGE_OFFSET would work on most archs,
but not necessarily on powerpc for example.
Johannes
> [ 32.285431] invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
> [ 32.285431] CPU 0
> [ 32.285431] Pid: 5643, comm: a.out Tainted: G W
> 3.5.0-next-20120727-sasha #504
> [ 32.285431] RIP: 0010:[<ffffffff810acd97>] [<ffffffff810acd97>]
> __phys_addr+0x57/0xa0
> [ 32.285431] RSP: 0018:ffff88000be67d68 EFLAGS: 00010213
> [ 32.285431] RAX: ffff87ffffffffff RBX: ffff88000d67cb00 RCX:
> 00000000000080d0
> [ 32.285431] RDX: 0000000000000071 RSI: ffff88000bfc8dc8 RDI:
> 0000000000001000
> [ 32.285431] RBP: ffff88000be67d68 R08: 0000000000000001 R09:
> ffff88000bfc8dc8
> [ 32.285431] R10: ffff88000bfc81f8 R11: 0000000000000002 R12:
> ffff88000bfc8dc8
> [ 32.285431] R13: 00007f26f80e6000 R14: ffff88000bf81000 R15:
> ffff88000bfc8dc8
> [ 32.285431] FS: 00007f26f80e8700(0000) GS:ffff88000d800000(0000)
> knlGS:0000000000000000
> [ 32.285431] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 32.285431] CR2: 00007f26f7c07d50 CR3: 000000000bfb8000 CR4:
> 00000000000406f0
> [ 32.285431] DR0: 0000000000000000 DR1: 0000000000000000 DR2:
> 0000000000000000
> [ 32.285431] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7:
> 0000000000000400
> [ 32.285431] Process a.out (pid: 5643, threadinfo ffff88000be66000, task
> ffff88000bf2b000)
> [ 32.285431] Stack:
> [ 32.285431] ffff88000be67d88 ffffffff81bb0737 ffff88000bfc95e0
> ffff88000bfc95f0
> [ 32.285431] ffff88000be67e48 ffffffff812163ae ffff88000d67cb00
> 0000000000000001
> [ 32.285431] 0000000000000000 0000000000000000 ffff88000be67dd8
> ffff88000bfc81f8
> [ 32.285431] Call Trace:
> [ 32.285431] [<ffffffff81bb0737>] mmap_kmem+0x27/0x90
> [ 32.285431] [<ffffffff812163ae>] mmap_region+0x35e/0x5f0
> [ 32.285431] [<ffffffff812168f9>] do_mmap_pgoff+0x2b9/0x350
> [ 32.285431] [<ffffffff8120120c>] ? vm_mmap_pgoff+0x6c/0xb0
> [ 32.285431] [<ffffffff81201224>] vm_mmap_pgoff+0x84/0xb0
> [ 32.285431] [<ffffffff8124f280>] ? fget_raw+0x260/0x260
> [ 32.285431] [<ffffffff81213d9e>] sys_mmap_pgoff+0x15e/0x190
> [ 32.285431] [<ffffffff8198ab2e>] ? trace_hardirqs_on_thunk+0x3a/0x3f
> [ 32.285431] [<ffffffff8106e4ed>] sys_mmap+0x1d/0x20
> [ 32.285431] [<ffffffff8361f6f9>] system_call_fastpath+0x16/0x1b
> [ 32.285431] Code: 00 00 00 00 eb fe 66 0f 1f 44 00 00 48 03 05 91 02 78 03
> eb 57 0f 1f 80 00 00 00 00 48 b8 ff ff ff ff ff 87 ff ff 48 39 c7 77 11 <0f>
> 0b 0f 1f 80 00 00 00 00 eb fe 66 0f 1f 44 00 00 48 b8 00 00
> [ 32.285431] RIP [<ffffffff810acd97>] __phys_addr+0x57/0xa0
> [ 32.285431] RSP <ffff88000be67d68>
>
> I could send a patch to do what I think it's supposed to be doing, but I find
> it odd since apparently /dev/kmem has been broken for a while now - which
> doesn't make sense.
>
> What am I missing?
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