[PATCH v3 0/3] alpha: stack fixes

[Ivan Kokshaysky]

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.

Patche 1 is preparatory; 2 - the main fix; 3 - fixes remaining
special cases.

Ivan.

[1] https://lore.kernel.org/rcu/CA+=Fv5R9NG+1SHU9QV9hjmavycHKpnNyerQ=Ei90G98ukRcRJA@xxxxxxxxxxxxxx/#r
[2] https://lore.kernel.org/rcu/alpine.DEB.2.21.2501130248010.18889@xxxxxxxxxxxxxxxxx/
[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@xxxxxxxxxxxxxxxxxxx>;
  update comment and commit message accordingly;
- cc'ed <stable@xxxxxxxxxxxxxxx> as older kernels ought to be fixed as well.

Changes in v3:
- patch #1 dropped for the time being;
- updated commit messages as Maciej suggested.
---
Ivan Kokshaysky (3):
  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/uapi/asm/ptrace.h |  2 ++
 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 ++--
 5 files changed, 19 insertions(+), 17 deletions(-)

-- 
2.47.2