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

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Message-ID: <20250129020439.rv4e25ppqoeo5y5u@jpoimboe>
Date: Tue, 28 Jan 2025 18:04:39 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Jens Remus <jremus@...ux.ibm.com>
Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>, 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,
	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 Tue, Jan 28, 2025 at 11:50:25AM +0100, Jens Remus wrote:
> On 28.01.2025 01:39, Andrii Nakryiko wrote:
> > On Fri, Jan 24, 2025 at 1:41 PM Josh Poimboeuf <jpoimboe@...nel.org> wrote:
> > > On Fri, Jan 24, 2025 at 10:02:46AM -0800, Andrii Nakryiko wrote:
> > > > On Tue, Jan 21, 2025 at 6:32 PM Josh Poimboeuf <jpoimboe@...nel.org> wrote:
> 
> > > > > +       UNSAFE_GET_USER_INC(info, cur, 1, Efault);
> > > > > +       offset_count = SFRAME_FRE_OFFSET_COUNT(info);
> > > > > +       offset_size  = offset_size_enum_to_size(SFRAME_FRE_OFFSET_SIZE(info));
> > > > > +       if (!offset_count || !offset_size)
> > > > > +               return -EFAULT;
> > > > > +
> > > > > +       if (cur + (offset_count * offset_size) > sec->fres_end)
> > > > 
> > > > offset_count * offset_size done in u8 can overflow, no? maybe upcast
> > > > to unsigned long or use check_add_overflow?
> 
> The maximum offset_count * offset_size is 15 * 4 = 60 if I am not wrong:
> 
> > > offset_size is <= 2 as returned by offset_size_enum_to_size().
> 
> SFrame V2 FRE offset sizes are either 1, 2, or 4 bytes.  This is also
> reflected in offset_size_enum_to_size().
> 
> > > offset_count is expected to be <= 3, enforced by the !offset_count check
> > > at the bottom.
> 
> SFrame V2 FRE offset count is 4 bits unsigned, so 0 <= offset_count <= 15.
> > > An overflow here would be harmless as it would be caught by the
> > > !offset_count anyway.  Though I also notice offset_count isn't big
> > > enough to hold the 2-byte SFRAME_FRE_OFFSET_COUNT() value.  Which is
> > > harmless for the same reason, but yeah I'll make offset_count an
> > > unsigned int.
> 
> As mentioned above the FRE offset count is 4 bits, not 2 bytes.  This is
> also reflected in SFRAME_FRE_OFFSET_COUNT().

You are right on both counts, not sure what I was smoking that day.

-- 
Josh