linux-kernel - Re: [PATCH v2] ARM: unwind: improve unwinders for noreturn case

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <ZfwmomjUwQdCefzh@shell.armlinux.org.uk>
Date: Thu, 21 Mar 2024 12:22:58 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: David Laight <David.Laight@...lab.com>
Cc: Ard Biesheuvel <ardb@...nel.org>,
	'Jiangfeng Xiao' <xiaojiangfeng@...wei.com>,
	"arnd@...db.de" <arnd@...db.de>,
	"keescook@...omium.org" <keescook@...omium.org>,
	"haibo.li@...iatek.com" <haibo.li@...iatek.com>,
	"angelogioacchino.delregno@...labora.com" <angelogioacchino.delregno@...labora.com>,
	"amergnat@...libre.com" <amergnat@...libre.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"douzhaolei@...wei.com" <douzhaolei@...wei.com>,
	"gustavoars@...nel.org" <gustavoars@...nel.org>,
	"jpoimboe@...nel.org" <jpoimboe@...nel.org>,
	"kepler.chenxin@...wei.com" <kepler.chenxin@...wei.com>,
	"kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
	"linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	"nixiaoming@...wei.com" <nixiaoming@...wei.com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"wangbing6@...wei.com" <wangbing6@...wei.com>,
	"wangfangpeng1@...wei.com" <wangfangpeng1@...wei.com>,
	"jannh@...gle.com" <jannh@...gle.com>,
	"willy@...radead.org" <willy@...radead.org>
Subject: Re: [PATCH v2] ARM: unwind: improve unwinders for noreturn case

On Thu, Mar 21, 2024 at 12:07:51PM +0000, David Laight wrote:
> From: Russell King
> > Sent: 21 March 2024 11:24
> > 
> > On Thu, Mar 21, 2024 at 10:22:30AM +0000, David Laight wrote:
> > > How aggressively does the compiler optimise 'noreturn' functions?
> > 
> > I've seen cases where the compiler emits a BL instruction as the very
> > last thing in the function, and nothing after it.
> 
> I've also seen the compiler defer generating a stack frame until
> after an initial conditional.

. which is why we pass -mno-sched-prolog to GCC.

> That might mean you can get the BL in the middle of a function
> but where the following instruction is for the 'no stack frame'
> side of the branch.
> That is very likely to break any stack offset calculations. 

No it can't. At any one point in the function, the stack has to be in
a well defined state, so that access to local variables can work, and
also the stack can be correctly unwound. If there exists a point in
the function body which can be reached where the stack could be in two
different states, then the stack can't be restored to the parent
context.

> > This is where the problem lies - because the link register value
> > created by the BL instruction will point to the instruction after the
> > BL which will _not_ part of the function that invoked the BL. That
> > will probably cause issues for the ELF unwinder, which means this
> > issue probably goes beyond _just_ printing the function name.
> 
> Isn't this already in the unwinder?
> A BL itself isn't going to fault with PC = next-instruction.

You are missing the fact that the PC can be the saved LR, and thus
can very well be the next instruction.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!