On 19 June 2017 at 17:03, Will Deacon <will.dea...@arm.com> wrote: > On Mon, Jun 19, 2017 at 04:37:24PM +0200, Ard Biesheuvel wrote: >> On arm64, the /dev/kmem driver barely works, given that it assumes that >> VMALLOC_START > PAGE_OFFSET, which is not the case on arm64. Due to the > > Probably worth being explicit about the failure mode here (the commit > message you had in your RFC was much better ;). >
Yeah, that's a fair point, I was being lazy :-) I will repost with the v1 commit log integrated >> dubious value of kmem on an architecture such as arm64, which is fairly >> new and is not as forgiving as other architectures when it comes to >> userland accesses to kernel memory using mismatched attributes*, let's >> disable it altogether if CONFIG_ARM64=y. >> >> * Mappings in the vmalloc region will be accessed using vread/vwrite when >> reading/writing to/from /dev/kmem, which will perform these accesses by >> kmap()'ing each page using writeback cacheable attributes. This will be >> correct in most cases, but this is not guaranteed. >> >> Signed-off-by: Ard Biesheuvel <ard.biesheu...@linaro.org> >> --- >> drivers/char/Kconfig | 2 ++ >> 1 file changed, 2 insertions(+) >> >> diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig >> index 31adbebf812e..8102ee7b3247 100644 >> --- a/drivers/char/Kconfig >> +++ b/drivers/char/Kconfig >> @@ -17,6 +17,8 @@ config DEVMEM >> >> config DEVKMEM >> bool "/dev/kmem virtual device support" >> + # On arm64, VMALLOC_START < PAGE_OFFSET, which confuses kmem read/write >> + depends on !ARM64 >> help >> Say Y here if you want to support the /dev/kmem device. The >> /dev/kmem device is rarely used, but can be used for certain >> -- >> 2.7.4 >>
