Re: [RFC PATCH v1 22/25] unwind_user/eh_frame/x86: Handle DRAP expressions

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Inner DRAP frames fail to unwind due to `state->topmost` restriction on `UNWIND_USER_RULE_REG_OFFSET_DEREF`.
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--- Patch [22]: [RFC PATCH v1 22/25] unwind_user/eh_frame/x86: Handle DRAP expressions ---
commit a2eefb14435ba994b91a768aaf44090977190c84
Author: Jens Remus <[email protected]>

unwind_user/eh_frame/x86: Handle DRAP expressions

GCC uses DRAP (Dynamic Realign Argument Pointer) when the stack needs
to be dynamically realigned. This patch adds x86-specific handling for
the CFA and FP DWARF expressions patterns used to describe how to unwind
through such frames.

> diff --git a/kernel/unwind/eh_frame.c b/kernel/unwind/eh_frame.c
> index 19e2bc96ddbc2..49e8a3e8d794a 100644
> --- a/kernel/unwind/eh_frame.c
> +++ b/kernel/unwind/eh_frame.c
[ ... ]
> @@ -1206,6 +1231,16 @@ static __always_inline int __find_frame_row(struct eh_frame_section *sec,
>  		frame->fp.regnum = ctx.state.reg_regnum[FP_IDX];
>  		frame->fp.offset = 0;
>  		break;
> +	case REG_REGISTER_OFFSET_DEREF:
> +		if (ctx.state.reg_regnum[FP_IDX] > UINT_MAX)
> +			return -EINVAL;
> +		if (ctx.state.reg_offset[FP_IDX] < INT_MIN ||
> +		    ctx.state.reg_offset[FP_IDX] > INT_MAX)
> +			return -EOPNOTSUPP;
> +		frame->fp.rule = UNWIND_USER_RULE_REG_OFFSET_DEREF;

[Severity: High]
Will this rule cause unwinding to fail for inner stack frames?

When an inner stack frame requires DRAP evaluation, the unwinder evaluates
UNWIND_USER_RULE_REG_OFFSET_DEREF in kernel/unwind/user.c:unwind_user_next_common()
like this:

	if (!state->topmost || unwind_user_get_reg(&fp, frame->fp.regnum))
		return -EINVAL;

Since inner frames are not topmost, this check will return -EINVAL and
abort the unwinding process. Could this truncate backtraces that pass
through dynamically realigned stack frames?

> +		frame->fp.regnum = ctx.state.reg_regnum[FP_IDX];
> +		frame->fp.offset = ctx.state.reg_offset[FP_IDX];
> +		break;

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=22