linux-kernel - Re: [PATCH v4 19/39] unwind_user/sframe: Add support for reading .sframe contents

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Message-ID: <20250207210614.nks6bxad4jhdulwg@jpoimboe>
Date: Fri, 7 Feb 2025 13:06:14 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Jens Remus <jremus@...ux.ibm.com>
Cc: x86@...nel.org, Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-kernel@...r.kernel.org, Indu Bhagat <indu.bhagat@...cle.com>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-perf-users@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	linux-toolchains@...r.kernel.org, Jordan Rome <jordalgo@...a.com>,
	Sam James <sam@...too.org>, linux-trace-kernel@...r.kernel.org,
	Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Florian Weimer <fweimer@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Weinan Liu <wnliu@...gle.com>, Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH v4 19/39] unwind_user/sframe: Add support for reading
 .sframe contents

On Wed, Feb 05, 2025 at 10:47:58AM +0100, Jens Remus wrote:
> UNSAFE_GET_USER_INC(ra_off, cur, offset_size, Efault);
> 
> With offset_size=1 expands into:
> 
> __UNSAFE_GET_USER_INC(/*to=*/ra_off, /*from=*cur, /*type=*/u8, /*label=*/Efault);
> 
> Expands into:
> 
> {
> 	u8 __to;
> 	unsafe_get_user(__to, (u8 __user *)cur, Efault);
> 	cur += sizeof(__to);
> 	ra_off = (typeof(ra_off))__to;
> }
> 
> The issue is that on the last line __to is u8 instead of s8 and thus
> u8 gets casted to s32, which is performed without sign extension.  __to
> would need to be s8 or get casted to s8 for sign extension to take
> place.

Ah, I get it now, thanks for spelling that out for me.

Here's what I have at the moment:

#define __UNSAFE_GET_USER_INC(to, from, type, label)			\
({									\
	type __to;							\
	unsafe_get_user(__to, (type __user *)from, label);		\
	from += sizeof(__to);						\
	to = __to;							\
})

#define __UNSAFE_GET_USER_INC(to, from, size, label, u_or_s)		\
({									\
	switch (size) {							\
	case 1:								\
		__UNSAFE_GET_USER_INC(to, from, u_or_s##8, label);	\
		break;							\
	case 2:								\
		__UNSAFE_GET_USER_INC(to, from, u_or_s##16, label);	\
		break;							\
	case 4:								\
		__UNSAFE_GET_USER_INC(to, from, u_or_s##32, label);	\
		break;							\
	default:							\
		dbg_sec_uaccess("%d: bad unsafe_get_user() size %u\n",	\
				__LINE__, size);			\
		return -EFAULT;						\
	}								\
})

#define UNSAFE_GET_USER_UNSIGNED_INC(to, from, size, label)		\
	__UNSAFE_GET_USER_INC(to, from, size, label, u)

#define UNSAFE_GET_USER_SIGNED_INC(to, from, size, label)		\
	__UNSAFE_GET_USER_INC(to, from, size, label, s)

#define UNSAFE_GET_USER_INC(to, from, size, label)				\
	_Generic(to,								\
		 u8:	UNSAFE_GET_USER_UNSIGNED_INC(to, from, size, label),	\
		 u16:	UNSAFE_GET_USER_UNSIGNED_INC(to, from, size, label),	\
		 u32:	UNSAFE_GET_USER_UNSIGNED_INC(to, from, size, label),	\
		 s8:	UNSAFE_GET_USER_SIGNED_INC(to, from, size, label),	\
		 s16:	UNSAFE_GET_USER_SIGNED_INC(to, from, size, label),	\
		 s32:	UNSAFE_GET_USER_SIGNED_INC(to, from, size, label))