Hi Eric,
Am 24.07.2021 um 10:31 schrieb Eric W. Biederman:
Unless we change fpsp040_die() to call force_sig(SIGSEGV).
Yes. I think we would probably need to have it also call get_signal and
all of that, because I don't think the very light call path for that
exception includes testing if signals are pending.
As far as I can see, there is a test for pending signals:
ENTRY(ret_from_exception)
.Lret_from_exception:
btst #5,%sp@(PT_OFF_SR) | check if returning to kernel
bnes 1f | if so, skip resched, signals
| only allow interrupts when we are really the last one on the
| kernel stack, otherwise stack overflow can occur during
| heavy interrupt load
andw #ALLOWINT,%sr
resume_userspace:
movel %curptr@(TASK_STACK),%a1
moveb %a1@(TINFO_FLAGS+3),%d0 | bits 0-7 of TINFO_FLAGS
jne exit_work | any bit set? -> exit_work
1: RESTORE_ALL
exit_work:
| save top of frame
movel %sp,%curptr@(TASK_THREAD+THREAD_ESP0)
lslb #1,%d0 | shift out TIF_NEED_RESCHED
jne do_signal_return | any remaining bit
(signal/notify_resume)? -> do_signal_return
pea resume_userspace
jra schedule
As long as TIF_NOTIFY_SIGNAL or TIF_SIGPENDING are set,
do_signal_return will be called.
I was going to say I don't think so, as my tracing of
the code lead in a couple of different directions. Upon closer
inspection all those paths either lead to fpsp_done or more
directly to ret_from_exception.
For anyone else who might want to trace the code, or for myself later on
when I forget. As best as I can figure the hardware exception vector
table is setup in: arch/m68k/kernel/vector.c
For the vectors in question it appears to be this chunk of code:
if (CPU_IS_040 && !FPU_IS_EMU) {
/* set up FPSP entry points */
asmlinkage void dz_vec(void) asm ("dz");
asmlinkage void inex_vec(void) asm ("inex");
asmlinkage void ovfl_vec(void) asm ("ovfl");
asmlinkage void unfl_vec(void) asm ("unfl");
asmlinkage void snan_vec(void) asm ("snan");
asmlinkage void operr_vec(void) asm ("operr");
asmlinkage void bsun_vec(void) asm ("bsun");
asmlinkage void fline_vec(void) asm ("fline");
asmlinkage void unsupp_vec(void) asm ("unsupp");
vectors[VEC_FPDIVZ] = dz_vec;
vectors[VEC_FPIR] = inex_vec;
vectors[VEC_FPOVER] = ovfl_vec;
vectors[VEC_FPUNDER] = unfl_vec;
vectors[VEC_FPNAN] = snan_vec;
vectors[VEC_FPOE] = operr_vec;
vectors[VEC_FPBRUC] = bsun_vec;
vectors[VEC_LINE11] = fline_vec;
vectors[VEC_FPUNSUP] = unsupp_vec;
}
Correct.
Which leads me to call traces that look like this:
hw
fline
fpsp_fline
mem_read
user_read
copyin
in_ea
<page-fault>
fpsp040_die
According to my understanding, you can't get a F-line exception on
68040. F-line is a coprocessor protocol violation, only raised when
there is no coprocessor present on the bus.
What we expect to get is any of the arithmetic exceptions, and the
'unsupported opcode' one (for those floating point instructions that the
68040 FPU does not implement).
In reality, it's probably the 'unsupported' exception we expect to hit
most often.
If that mem_read returns it can be followed by
not_mvcr
real_fline
ret_from_exception
Or it can be followed by
fix_con
uni_2
gen_except
do_clean
finish_up
fpsp_done
ret_from_exception
The way the code is structured it is actively incorrect to return from
fpsp040_die, as the code does not know what to do if it reads a byte
from userspace and there is nothing there.
Correct - my hope is that upon return from the FPU exception (that
continued after a dodgy read or write), we get the signal delivered
and will die then.
Yes. That does look like a good strategy.
I am wondering if there are values we can return that will make the
path out of the exit routine more deterministic.
I doubt it - maybe 'preloading' the register used for the read with
something invalid as floating point instruction or data might force
another exception more readily, but that's just speculation on my part.
I have played with that a little bit today, but it doesn't look like I
am going to have time to put together any kind of real patch today.
I've attached a corrected version of my patch to supply the required
stack frame - this ought to make use of do_exit() safe. Still working on
a way to exercise this code path. Let's think about ways to use signals
once I've succeeded to do that.
Cheers,
Michael
Simply modifying fpsp040_die to call force_sigsegv(SIGSEGV) should be
enough to trigger a signal (no call stack work needed if we remove
do_exit). The tricky bit is what value do we want to fake when we
can not read anything from userspace. For a write fault we should just
be able to skip the write entirely.
In both cases we probably should break out of the loop prematurely. But
I don't know if that is necessary.
The lazy strategy would be to copy the ifpsp060 code and simply oops
the kernel if the read or write of userspace gets a page fault.
So instead of handling -EFAULT like most pieces of kernel code the code
just immediately calls do_exit, and does not even attempt to handle
the error.
That is not my favorite strategy at all, but I suspect it isn't worth
it, or safe to update the skeleton.S to handle errors. Especially as we
have not even figured out how to test that code yet.
That's bothering me more than a little, but I need to find out whether
the emulator even handles FPU exceptions correctly ...
As a fallback plan we can following the lead of ifpsp060/os.S and simply
not catch the kernel triggered page fault, and let
arch/m68k/mm/fault.c:send_fault_sig() return a kernel oops. It is not
ideal as it allows userspace to trigger a kernel oops, but it does at
least keep the kernel in a consistent state.
diff --git a/arch/m68k/fpsp040/skeleton.S b/arch/m68k/fpsp040/skeleton.S
index a8f41615d94a..4c6c4b07ef38 100644
--- a/arch/m68k/fpsp040/skeleton.S
+++ b/arch/m68k/fpsp040/skeleton.S
@@ -479,7 +479,6 @@ copyout:
| movec %d1,%DFC | set dfc for user data space
moreout:
moveb (%a0)+,%d1 | fetch supervisor byte
-out_ea:
movesb %d1,(%a1)+ | write user byte
dbf %d0,moreout
rts
@@ -493,21 +492,9 @@ copyin:
| SFC is already set
| movec %d1,%SFC | set sfc for user space
morein:
-in_ea:
movesb (%a0)+,%d1 | fetch user byte
moveb %d1,(%a1)+ | write supervisor byte
dbf %d0,morein
rts
- .section .fixup,#alloc,#execinstr
- .even
-1:
- jbra fpsp040_die
-
- .section __ex_table,#alloc
- .align 4
-
- .long in_ea,1b
- .long out_ea,1b
-
|end
diff --git a/arch/m68k/kernel/traps.c b/arch/m68k/kernel/traps.c
index e2a6f3556211..3ec6ae1bdaf9 100644
--- a/arch/m68k/kernel/traps.c
+++ b/arch/m68k/kernel/traps.c
@@ -1144,15 +1144,6 @@ asmlinkage void set_esp0(unsigned long ssp)
current->thread.esp0 = ssp;
}
-/*
- * This function is called if an error occur while accessing
- * user-space from the fpsp040 code.
- */
-asmlinkage void fpsp040_die(void)
-{
- do_exit(SIGSEGV);
-}
-
#ifdef CONFIG_M68KFPU_EMU
asmlinkage void fpemu_signal(int signal, int code, void *addr)
{
Eric
>From 737b74a376f0b3da09ba7cb088e99c2c85b7405c Mon Sep 17 00:00:00 2001
From: Michael Schmitz <schmitzmic@xxxxxxxxx>
Date: Sun, 18 Jul 2021 10:31:42 +1200
Subject: [PATCH] m68k/fpsp040 - save full stack frame before calling
fpsp040_die
The FPSP040 floating point support code does not know how to
handle user space access faults gracefully, and just calls
do_exit(SIGSEGV) indirectly on these faults to abort.
do_exit() may stop if traced, and needs a full stack frame
available to avoid exposing kernel data.
Add the current stack frame before calling do_exit() from the
fpsp040 user access exception handler. Top of stack frame saved
to task->thread.esp0 as is done for system calls.
Unwind the stack frame and return to caller once done, in case
do_exit() is replaced by force_sig() later on. Note that this
will allow the current exception handler to continue with
incorrect state, but the results will never make it to the
calling user program which is terminated by SYSSIGV upon return
from exception.
CC: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Signed-off-by: Michael Schmitz <schmitzmic@xxxxxxxxx>
---
arch/m68k/fpsp040/skeleton.S | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/arch/m68k/fpsp040/skeleton.S b/arch/m68k/fpsp040/skeleton.S
index a8f4161..1cbc52b 100644
--- a/arch/m68k/fpsp040/skeleton.S
+++ b/arch/m68k/fpsp040/skeleton.S
@@ -502,7 +502,14 @@ in_ea:
.section .fixup,#alloc,#execinstr
.even
1:
+
+ SAVE_ALL_INT
+ | save top of frame
+ movel %sp,%curptr@(TASK_THREAD+THREAD_ESP0)
+ SAVE_SWITCH_STACK
jbra fpsp040_die
+ lea 44(%sp),%sp
+ rts
.section __ex_table,#alloc
.align 4
--
2.7.4
