Re: [PATCH v2 2/7] mips: fix mips_get_syscall_arg() for O32 and N32

[Maciej W. Rozycki]

On Mon, 13 Jan 2025, Dmitry V. Levin wrote:

> Fix the following get_syscall_info test assertion on mips O32:
>   # get_syscall_info.c:218:get_syscall_info:Expected exp_args[5] (3134521044) == info.entry.args[4] (4911432)
>   # get_syscall_info.c:219:get_syscall_info:wait #1: entry stop mismatch
> 
> Fix the following get_syscall_info test assertion on mips64 O32 and mips64 N32:
>   # get_syscall_info.c:209:get_syscall_info:Expected exp_args[2] (3134324433) == info.entry.args[1] (18446744072548908753)
>   # get_syscall_info.c:210:get_syscall_info:wait #1: entry stop mismatch

 How did you produce these results?

> This makes ptrace/get_syscall_info selftest pass on mips O32,
> mips64 O32, and mips64 N32.
> 
> Signed-off-by: Dmitry V. Levin <ldv@xxxxxxxxx>
> ---
> 
> Note that I'm not a MIPS expert, so I cannot tell why the get_user()
> approach doesn't work for O32.  Also, during experiments I discovered that
> regs->pad0 approach works for O32, but why it works remains a mystery.

 The patch is definitely broken, the calling convention is the same 
between n32 and n64: 64-bit arguments in $4 through $11 registers as 
required, and your change makes n32 truncate arguments to 32 bits.

 The regs->pad0 approach works due to this piece:

	/*
	 * Ok, copy the args from the luser stack to the kernel stack.
	 */

	.set    push
	.set    noreorder
	.set	nomacro

load_a4: user_lw(t5, 16(t0))		# argument #5 from usp
load_a5: user_lw(t6, 20(t0))		# argument #6 from usp
load_a6: user_lw(t7, 24(t0))		# argument #7 from usp
load_a7: user_lw(t8, 28(t0))		# argument #8 from usp
loads_done:

	sw	t5, 16(sp)		# argument #5 to ksp
	sw	t6, 20(sp)		# argument #6 to ksp
	sw	t7, 24(sp)		# argument #7 to ksp
	sw	t8, 28(sp)		# argument #8 to ksp
	.set	pop

	.section __ex_table,"a"
	PTR_WD	load_a4, bad_stack_a4
	PTR_WD	load_a5, bad_stack_a5
	PTR_WD	load_a6, bad_stack_a6
	PTR_WD	load_a7, bad_stack_a7
	.previous

in arch/mips/kernel/scall32-o32.S (and arch/mips/kernel/scall64-o32.S has 
analogous code to adapt to the native n64 calling convention instead), but 
this doesn't seem to me to be the correct approach here.  At first glance 
`mips_get_syscall_arg' does appear fine as it is, so I'd like to know how 
you obtained your results.

  Maciej