ARM: add support for IRQ stacks

Compared to user space, the kernel's task stacks are tiny and
inflexible, as we don't grow them dynamically using demand paging. This
is the reason we tend to obsess about functions with disproportionately
large stack frames, given that it is hard to predict statically how
calls to those functions may combine at runtime, and exhaust the stack
and crash the kernel.

This becomes even less predictable when taking interrupt handling into
account, as their handlers are normally executed on the stack of the
task that was interrupted, regardless of how deep its call stack was at
the time of the interruption. To decouple these, and reduce the risk of
hitting a pathological worst case where IRQ handling and the task below
it happen to exhaust the available stack space when combined, we can
switch to a different stack when taking interrupts, similar to how this
is done already on some other architectures. This series implements this
approach for ARM.

Note that a good chunk of the changes below are related to supporting
non-contiguous call stacks, which is also relevant in the context of
vmap'ed stacks, which use a separate overflow stack to handle stack
overflows. The changes preserve all functionality related to walking the
call stack and dumping exception stacks and register contents.

Changes since v3:
- rebase onto v5.16-rc1
- dropped patch abstracting the call to (*handle_arch_irq)() from
  entry.S, which is no longer needed due to cross-arch rework that
  landed during the merge window
- add some acks from Nick

Changes since v2:
- improve Clang support, by emitting code that is compatible with its
  frame pointer unwinder if that is the unwinder being used;
- add acks from Arnd and Linus (thanks!)

Changes since v1:
- drop the first bugfix patch, which has been queued as a fix in the
  meantime;
- preserve/restore FP in the irq_handler entry code;
- add missing include to arch/arm/kernel/irq.c to silence warnings about
  missing prototypes.

Patch #1 removes some code that I spotted that is no longer used.

Patch #2 introduces a macro that will be used later in the series to
emit the optimal indirect call sequence for older and newer cores.

Patch #3 updates the unwind info based unwinder so it can deal with call
stacks that are non-contiguous.

Patch #4 exports dump_mem() to other compilation units so the ARM
unwinder can call it directly. This is needed by the next patch.

Patch #5 refactors the ARM unwinder to dump the exception stack from a
context where it is able to figure out if it lives on the current stack
or on the task stack.

Patch #6 fixes an issue in the Clang frame pointer unwinder, which may
get into an endless recursive fault if any of the stack frames have a
bogus value for the link register.

Patch #7 implements the actual IRQ stacks support, by allocating one for
each CPU, and adding the code to switch to it in the IRQ entry path. It
also contains some related changes to allow the frame pointer based
unwinder to deal with the new situation.

Patch #8 modifies call_with_stack() so both the frame pointer unwinder
as well as the ARM unwinder know how to deal with it.

Patch #9 adds the IRQ stack switching for softIRQ handling initiated
from task context.

Cc: Russell King <linux@armlinux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Kees Cook <keescook@chromium.org>
Cc: Keith Packard <keithpac@amazon.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nicolas Pitre <nico@fluxnic.net>

Ard Biesheuvel (9):
  ARM: remove some dead code
  ARM: assembler: introduce bl_r macro
  ARM: unwind: support unwinding across multiple stacks
  ARM: export dump_mem() to other objects
  ARM: unwind: dump exception stack from calling frame
  ARM: backtrace-clang: avoid crash on bogus frame pointer
  ARM: implement IRQ stacks
  ARM: call_with_stack: add unwind support
  ARM: run softirqs on the per-CPU IRQ stack

 arch/arm/Kconfig                         |  6 +++
 arch/arm/include/asm/assembler.h         | 19 +++++++
 arch/arm/include/asm/entry-macro-multi.S | 24 ---------
 arch/arm/include/asm/smp.h               |  5 --
 arch/arm/include/asm/stacktrace.h        | 12 +++++
 arch/arm/kernel/entry-armv.S             | 54 ++++++++++++++++++--
 arch/arm/kernel/irq.c                    | 37 ++++++++++++++
 arch/arm/kernel/smp.c                    |  5 --
 arch/arm/kernel/traps.c                  | 25 ++++++---
 arch/arm/kernel/unwind.c                 | 33 ++++++++----
 arch/arm/lib/backtrace-clang.S           | 14 +++--
 arch/arm/lib/backtrace.S                 |  8 +++
 arch/arm/lib/call_with_stack.S           | 33 +++++++++---
 13 files changed, 211 insertions(+), 64 deletions(-)

-- 
2.30.2


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