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SubjectRe: objtool/ORC generation for noreturn functions
On Wed, Jan 13, 2021 at 10:39:53PM +0000, David Laight wrote:
> > On Wed, Jan 13, 2021 at 11:44:22AM +0100, vanessa.hack@fau.de wrote:
> > > Hi,
> > > I am currently writing my final thesis at university on the topic of stack
> > > unwinding. My goal is to implement and evaluate stack unwinders for
> > > research operating system ports to x86 32 and 64 bit architectures and
> > > SPARC V8.
> > > For the x86 ports I chose ORC as unwinding format due to its simplicity
> > > and reliability. So far, it works quite well (although I've ran into some
> > > minor issues with objtool as the research OS is written in C++).
> > > But now I have some problems with functions that are explicitly marked as
> > > noreturn with the [[noreturn]] attribute, all following unwinding steps
> > > are unreliable. I have read in the objtool documentation that such
> > > functions have to be added to the objtool global_noreturn array.
> > > Unfortunately, I do not understand the purpose of that array and the
> > > intended ORC behaviour for noreturn functions. Are the unwinding steps
> > > that follow a noreturn intended to be unreliable?
>
> There was an 'interesting' unwinder I saw a few years ago.
> (Which couldn't handle 'noreturn' functions.)
>
> The idea is to follow forwards through the code while keeping
> track of %sp and %fp until a return instruction is found.
> You need to be able to detect loops, and then continue from
> the other target of an earlier conditional branch.
> Provided function calls don't change %sp they can be ignored.
> If the %fp isn't used as a frame pointer it won't get reloaded
> into %sp so it doesn't matter
>
> This works (most of the time) with no debug info and no symbol
> table.

Almost sounds like an in-kernel version of objtool :-)

--
Josh

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