The problem is that GCC expects 16-byte alignment of the incoming stack
since early 2004, as Maciej found out [1]:
Having actually dug speculatively I can see that the psABI was changed in
GCC 3.5 with commit e5e10fb4a350 ("re PR target/14539 (128-bit long double
improperly aligned)") back in Mar 2004, when the stack pointer alignment
was increased from 8 bytes to 16 bytes, and arch/alpha/kernel/entry.S has
various suspicious stack pointer adjustments, starting with SP_OFF which
is not a whole multiple of 16.
Also, as Magnus noted, "ALPHA Calling Standard" [2] required the same:
D.3.1 Stack Alignment
This standard requires that stacks be octaword aligned at the time a
new procedure is invoked.
However:
- the "normal" kernel stack is always misaligned by 8 bytes, thanks to
the odd number of 64-bit words in 'struct pt_regs', which is the very
first thing pushed onto the kernel thread stack;
- syscall, fault, interrupt etc. handlers may, or may not, receive aligned
stack depending on numerous factors.
Somehow we got away with it until recently, when we ended up with
a stack corruption in kernel/smp.c:smp_call_function_single() due to
its use of 32-byte aligned local data and the compiler doing clever
things allocating it on the stack.
This adds padding between the PAL-saved and kernel-saved registers
so that 'struct pt_regs' have an even number of 64-bit words.
This makes the stack properly aligned for most of the kernel
code, except two handlers which need special threatment.
Note: struct pt_regs doesn't belong in uapi/asm; this should be fixed,
but let's put this off until later.
Link: https://lore.kernel.org/rcu/alpine.DEB.2.21.2501130248010.18889@xxxxxxxxxxxxxxxxx/ [1]
Link: https://bitsavers.org/pdf/dec/alpha/Alpha_Calling_Standard_Rev_2.0_19900427.pdf [2]
Cc: stable@xxxxxxxxxxxxxxx
Reviewed-by: Maciej W. Rozycki <macro@xxxxxxxxxxx>
Tested-by: Magnus Lindholm <linmag7@xxxxxxxxx>
Signed-off-by: Ivan Kokshaysky <ink@unseen.parts>
---
arch/alpha/include/uapi/asm/ptrace.h | 2 ++
1 file changed, 2 insertions(+)
diff --git a/arch/alpha/include/uapi/asm/ptrace.h b/arch/alpha/include/uapi/asm/ptrace.h
index 5ca45934fcbb..72ed913a910f 100644
--- a/arch/alpha/include/uapi/asm/ptrace.h
+++ b/arch/alpha/include/uapi/asm/ptrace.h
@@ -42,6 +42,8 @@ struct pt_regs {
unsigned long trap_a0;
unsigned long trap_a1;
unsigned long trap_a2;
+/* This makes the stack 16-byte aligned as GCC expects */
+ unsigned long __pad0;
/* These are saved by PAL-code: */
unsigned long ps;
unsigned long pc;
--
2.47.2
