linux-kernel - [PATCH v3 2/3] alpha: make stack 16-byte aligned (most cases)

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Message-Id: <20250204223524.6207-3-ink@unseen.parts>
Date: Tue,  4 Feb 2025 23:35:23 +0100
From: Ivan Kokshaysky <ink@...een.parts>
To: Richard Henderson <richard.henderson@...aro.org>,
	Matt Turner <mattst88@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Arnd Bergmann <arnd@...db.de>,
	"Paul E. McKenney" <paulmck@...nel.org>
Cc: "Maciej W. Rozycki" <macro@...am.me.uk>,
	Magnus Lindholm <linmag7@...il.com>,
	linux-alpha@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: [PATCH v3 2/3] alpha: make stack 16-byte aligned (most cases)

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@angie.orcam.me.uk/ [1]
Link: https://bitsavers.org/pdf/dec/alpha/Alpha_Calling_Standard_Rev_2.0_19900427.pdf [2]

Cc: stable@...r.kernel.org
Reviewed-by: Maciej W. Rozycki <macro@...am.me.uk>
Tested-by: Magnus Lindholm <linmag7@...il.com>
Signed-off-by: Ivan Kokshaysky <ink@...een.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