[RFC PATCH 0/3] arm64: Implement reliable stack trace

[Josh Poimboeuf]

Hi all,

I know this is an old patch set, but I'm not able to see the context,
because I'm not subcribed to the ML.  For future patch sets can you also
add live-patching and lkml?

Are we still considering the shadow stack thing?  Just wondering why
we're talking about objtool again.

On Thu, Jan 28, 2021 at 02:22:50PM +0000, Mark Brown wrote:
> On Wed, Jan 27, 2021 at 01:54:21PM -0600, Madhavan T. Venkataraman wrote:
> > Instead of failing the kernel build, can we just mark the functions as:
> 
> > 	FP	Functions that have proper FP code
> > 	no-FP	Functions that don't
> 
> > May be, we can add an "FP" flag in the symbol table entry for this.
> 
> > Then, the unwinder can check the functions it encounters in the stack trace and
> > inform the caller if it found any no-FP functions. The caller of the unwinder can
> > decide what he wants to do with that information.
> > 
> > 	- the caller can ignore it
> > 
> > 	- the caller can print the stack trace with a warning that no-FP functions
> > 	  were found
> > 
> > 	- if the caller is livepatch, the caller can retry until the no-FP functions
> > 	  disappear from the stack trace. This way, we can have live patching even
> > 	  when some of the functions in the kernel are no-FP.
> > 
> > Does this make any sense? Is this acceptable? What are the pitfalls?

Why not just fix the reported no-FP functions?

> > If we can do this, the unwinder could detect cases such as:
> 
> > - If gcc thinks that a function is a leaf function but the function contains
> >   inline assembly code that calls another function.
> 
> > - If a call to a function bounces through some intermediate code such as a
> >   trampoline.
> 
> > - etc.
> 
> > For specific no-FP functions, the unwinder might be able to deduce the original
> > caller. In these cases, the stack trace would still be reliable. For all the others,
> > the stack trace would be considered unreliable.
> 
> I'm not entirely sure I see the distinction from the current situation
> here?
> 
> > Compiler instead of objtool
> > ===========================
> > 
> > If the above suggestion is acceptable, I have another suggestion.
> > 
> > It is a lot of work for every new architecture to add frame pointer verification
> > support in objtool. Can we get some help from the compiler?
> > 
> > The compiler knows which C functions it generates the FP prolog and epilog for. It can
> > mark those functions as FP. As for assembly functions, kernel developers could manually
> > annotate functions that have proper FP code. The compiler/assembler would mark them
> > as FP. Only a small subset of assembly functions would even have FP prolog and epilog.
> 
> > Is this acceptable? What are the pitfalls?
> 
> If we're trusting the compiler we can probably just do that without any
> explicit support from the compiler - it should be doing the standard
> stuff unless we explicitly ask it to and if it isn't then it might be a
> result of a mismatch in assumptions rather than a deliberate decision to
> do something non-standard.  My understanding with objtool is that a big
> part of the idea is to provide a static check that the binary we end up
> with matches the assumptions that we are making so the fact that it's a
> separate implementation is important.

For C code, even if we trusted the compiler (which we don't), we still
have inline asm which the compiler doesn't have any visibility to, which
is more than capable of messing up frame pointers (we had several cases
of this in x86).

Getting the assembler to annotate which functions are FP could be
interesting but:

a) good luck getting the assembler folks to do that; they tend to be
   insistent on being ignorant of code semantics;

b) assembly often resembles spaghetti and the concept of a function is
   quite fluid; in fact many functions aren't annotated as such.

-- 
Josh