Re: [PATCH v2 0/4] alpha: stack fixes

[John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>]

Hi Ivan,

On Fri, 2025-01-31 at 11:41 +0100, Ivan Kokshaysky wrote:
> This series fixes oopses on Alpha/SMP observed since kernel v6.9. [1]
> Thanks to Magnus Lindholm for identifying that remarkably longstanding
> bug.
> 
> The problem is that GCC expects 16-byte alignment of the incoming stack
> since early 2004, as Maciej found out [2]:
>   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" [3] 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.
> 
> Patches 1-2 are preparatory; 3 - the main fix; 4 - fixes remaining
> special cases.
> 
> Ivan.
> 
> [1] https://lore.kernel.org/rcu/CA+=Fv5R9NG+1SHU9QV9hjmavycHKpnNyerQ=Ei90G98ukRcRJA@mail.gmail.com/#r
> [2] https://lore.kernel.org/rcu/alpine.DEB.2.21.2501130248010.18889@angie.orcam.me.uk/
> [3] https://bitsavers.org/pdf/dec/alpha/Alpha_Calling_Standard_Rev_2.0_19900427.pdf
> ---
> Changes in v2:
> - patch #1: provide empty 'struct pt_regs' to fix compile failure in libbpf,
>   reported by John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>;
>   update comment and commit message accordingly;
> - cc'ed <stable@vger.kernel.org> as older kernels ought to be fixed as well.
> ---
> Ivan Kokshaysky (4):
>   alpha/uapi: do not expose kernel-only stack frame structures
>   alpha: replace hardcoded stack offsets with autogenerated ones
>   alpha: make stack 16-byte aligned (most cases)
>   alpha: align stack for page fault and user unaligned trap handlers
> 
>  arch/alpha/include/asm/ptrace.h      | 64 ++++++++++++++++++++++++++-
>  arch/alpha/include/uapi/asm/ptrace.h | 65 ++--------------------------
>  arch/alpha/kernel/asm-offsets.c      |  4 ++
>  arch/alpha/kernel/entry.S            | 24 +++++-----
>  arch/alpha/kernel/traps.c            |  2 +-
>  arch/alpha/mm/fault.c                |  4 +-
>  6 files changed, 83 insertions(+), 80 deletions(-)

Thanks, I'm testing the v2 series of the patches now.

Adrian

-- 
 .''`.  John Paul Adrian Glaubitz
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