LoongArch: Add identifier names to arguments of die() declaration

Add identifier names to arguments of die() declaration in ptrace.h
to fix the following checkpatch warnings:

WARNING: function definition argument 'const char *' should also have an identifier name
WARNING: function definition argument 'struct pt_regs *' should also have an identifier name

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: Do not kill the task in die() if notify_die() returns NOTIFY_STOP

If notify_die() returns NOTIFY_STOP, honor the return value from the
handler chain invocation in die() and return without killing the task
as, through a debugger, the fault may have been fixed. It makes sense
even if ignoring the event will make the system unstable: by allowing
access through a debugger it has been compromised already anyway. It
makes our port consistent with x86, arm64, riscv and csky.

Commit 20c0d2d44029 ("[PATCH] i386: pass proper trap numbers to die
chain handlers") may be the earliest of similar changes.

Link: https://lore.kernel.org/r/43DDF02E.76F0.0078.0@novell.com/
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: Add support for function error injection

Inspired by the commit 42d038c4fb00f ("arm64: Add support for function
error injection") and the commit ee55ff803b383 ("riscv: Add support for
function error injection"), this patch supports function error injection
for LoongArch.

Mainly implement two functions:
(1) regs_set_return_value() which is used to overwrite the return value,
(2) override_function_with_return() which is used to override the probed
function returning and jump to its caller.

Here is a simple test under CONFIG_FUNCTION_ERROR_INJECTION and
CONFIG_FAIL_FUNCTION:

# echo sys_clone > /sys/kernel/debug/fail_function/inject
# echo 100 > /sys/kernel/debug/fail_function/probability
# dmesg
bash: fork: Invalid argument
# dmesg
...
FAULT_INJECTION: forcing a failure.
name fail_function, interval 1, probability 100, space 0, times 1
...
Call Trace:
[<90000000002238f4>] show_stack+0x5c/0x180
[<90000000012e384c>] dump_stack_lvl+0x60/0x88
[<9000000000b1879c>] should_fail_ex+0x1b0/0x1f4
[<900000000032ead4>] fei_kprobe_handler+0x28/0x6c
[<9000000000230970>] kprobe_breakpoint_handler+0xf0/0x118
[<90000000012e3e60>] do_bp+0x2c4/0x358
[<9000000002241924>] exception_handlers+0x1924/0x10000
[<900000000023b7d0>] sys_clone+0x0/0x4
[<90000000012e4744>] do_syscall+0x7c/0x94
[<9000000000221e44>] handle_syscall+0xc4/0x160

Tested-by: Hengqi Chen <hengqi.chen@gmail.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: Simulate branch and PC* instructions

According to LoongArch Reference Manual, simulate branch and PC*
instructions, this is preparation for later patch.

Link: https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html#branch-instructions
Link: https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html#_pcaddi_pcaddu121_pcaddu18l_pcalau12i

Tested-by: Jeff Xie <xiehuan09@gmail.com>
Co-developed-by: Jinyang He <hejinyang@loongson.cn>
Signed-off-by: Jinyang He <hejinyang@loongson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: ptrace: Add hardware single step support

Use the generic ptrace_resume code for PTRACE_SYSCALL, PTRACE_CONT,
PTRACE_KILL and PTRACE_SINGLESTEP handling. This implies defining
arch_has_single_step() and implementing the user_enable_single_step()
and user_disable_single_step() functions.

LoongArch cannot do hardware single-stepping per se, the hardware
single-stepping it is achieved by configuring the instruction fetch
watchpoints (FWPS) and specifies that the next instruction must trigger
the watch exception by setting the mask bit. In some scenarios
CSR.FWPS.Skip is used to ignore the next hit result, avoid endless
repeated triggering of the same watchpoint without canceling it.

Signed-off-by: Qing Zhang <zhangqing@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: ptrace: Add function argument access API

Add regs_get_argument() which returns N th argument of the function
call, This enables ftrace kprobe events to access kernel function
arguments via $argN syntax for later use.

E.g.:
echo 'p bio_add_page arg1=$arg1' > kprobe_events
bash: echo: write error: Invalid argument

Signed-off-by: Qing Zhang <zhangqing@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: Use flexible-array member instead of zero-length array

Eliminate the following coccicheck warning:
./arch/loongarch/include/asm/ptrace.h:32:15-21: WARNING use flexible-array member instead

Reviewed-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Yushan Zhou <katrinzhou@tencent.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: Remove unused kernel stack padding

The current LoongArch kernel stack is padded as if obeying the MIPS o32
calling convention (32 bytes), signifying the port's MIPS lineage but no
longer making sense. Remove the padding for clarity.

Reviewed-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Jinyang He <hejinyang@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: Add process management

Add process management support for LoongArch, including: thread info
definition, context switch and process tracing.

Reviewed-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>