From mboxrd@z Thu Jan 1 00:00:00 1970 From: anfei.zhou@gmail.com (anfei) Date: Thu, 22 Apr 2010 20:29:01 +0800 Subject: kernel virtual memory access (from app) does not generatesegfault In-Reply-To: <000001cae20a$6ab7c820$4044010a@Emea.Arm.com> References: <20100420192813.GA29831@n2100.arm.linux.org.uk> <20100420223106.GQ11723@shareable.org> <20100420224108.GA1432@n2100.arm.linux.org.uk> <000001cae144$4281a9a0$4044010a@Emea.Arm.com> <20100421193534.GC26616@n2100.arm.linux.org.uk> <20100421214447.GK26616@n2100.arm.linux.org.uk> <20100421215425.GL26616@n2100.arm.linux.org.uk> <000001cae20a$6ab7c820$4044010a@Emea.Arm.com> Message-ID: <20100422122901.GA5126@desktop> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Thu, Apr 22, 2010 at 11:56:09AM +0100, Dave P. Martin wrote: > Hi there, > > > -----Original Message----- > > From: Nicolas Pitre [mailto:nico at fluxnic.net] > > Sent: 21 April 2010 23:59 > > To: Russell King - ARM Linux > > Cc: linux-arm-kernel at lists.infradead.org; anfei; Jamie > > Lokier; Dave P Martin; Ben Dooks > > Subject: Re: kernel virtual memory access (from app) does not > > generatesegfault > > > > On Wed, 21 Apr 2010, Russell King - ARM Linux wrote: > > > > > On Wed, Apr 21, 2010 at 10:44:47PM +0100, Russell King - > > ARM Linux wrote: > > > > On Wed, Apr 21, 2010 at 05:24:51PM -0400, Nicolas Pitre wrote: > > > > > On Wed, 21 Apr 2010, Russell King - ARM Linux wrote: > > > > > > > > > > > On Wed, Apr 21, 2010 at 12:17:41PM +0100, Dave P. > > Martin wrote: > > > > > > > > > > > > > > > > > > > > > > -----Original Message----- > > > > > > > > From: Russell King - ARM Linux > > > > > > > > [mailto:linux at arm.linux.org.uk] > > > > > > > > Sent: 20 April 2010 23:41 > > > > > > > > To: Jamie Lokier > > > > > > > > Cc: Ben Dooks; anfei; Dave P Martin; > > > > > > > > linux-arm-kernel at lists.infradead.org > > > > > > > > Subject: Re: kernel virtual memory access (from app) does > > > > > > > > not generatesegfault > > > > > > > > > > > > > > [...] > > > > > > > > > > > > > > > The difference between instruction faults and > > data faults is > > > > > > > > that we always interpret instruction faults on pre-ARMv6 > > > > > > > > CPUs as a 'translation fault' rather than a > > permission fault > > > > > > > > since they can't tell us what the problem was. > > > > > > > > > > > > > > Note that my observations were on an armv7 kernel. > > Should we > > > > > > > still hit the same bit of code in this case, or > > have I misdiagnosed the problem? > > > > > > > > > > > > If it was ARMv7, we should be reading the IFSR, which > > should be > > > > > > telling us that there's a permission fault trying to read > > > > > > instructions from 0xc0000000. > > > > > > > > > > > > If changing do_translation_fault() on a recent kernel > > fixes your > > > > > > problem, something's going wrong. Any chance you > > could add some > > > > > > debugging to > > > > > > do_PrefetchAbort() so that when you see your test program > > > > > > running (eg, if (strcmp(current->comm, "progname") == > > 0) { ... > > > > > > }) you could dump out the values of ifsr and addr please? > > > > > > > > > > If I remember right, the original bug report mentioned ARM926. > > > > > > > > So here we go again with confusion raining. > > > > > > > > Someone please tell me _definitively_ _what_ is being > > seen on _what_ > > > > CPU, and separate the two issues into two different threads. I'm > > > > going to ignore any further comments on this issue until that's > > > > done. Life is too short to try to work this out on my own. > > > > > > Actually, no, you're creating the confusion; this > > sub-thread is about > > > the behaviour on ARMv7, as a completely separate subject > > from ARM926. > > > > It is well possible that I missed the subject transition. > > > > The only person who provided a test program is Sasha Sirotkin > > who said: > > > > On Tue, 20 Apr 2010, Sasha Sirotkin wrote: > > > > > P.S. My kernel is 2.6.32.7 and the CPU is ARM926EJ-S rev 5 (v5l) > > > > Message-id: <4BCD7076.9030802@browserseal.com> > > > > Only later did Dave P. Martin mention having made similar > > observations on an ARMv7. > > To clarify: > > * I haven't tested this on 926 myself > * On armv7, I have observed the problem only on *old* kernels > (<2.6.32; which lack any of the patches under discussion) > * Using 2.6.34-rc1 (from rmk's versatile branch) on armv7, I get the > expected SEGV when userspace tries to execute >= TASK_SIZE > > so... > * Sasha's problem is caused by a problem in the current kernel on > 926. > * My problem relates to v7 and has already been fixed (but isn't > fixed in the Ubuntu kernels yet) > > The test case was > > int main(void) > { > ((void (*)(void))0xc0000000)(); > return 0; > } > I did a test on arm926 using QEMU with the latest kernel (just pull from git.kernel.org). Without checking user_mode, this test case will continue to trigger do_translation_fault with address 0xc0000000, so I think that two-liner patch is necessary. With it, the case will get SIGSEGV, and the system seems running well. Regards, Anfei. > Hope this makes things clearer. > > Cheers > ---Dave > >