[PATCH 02/10] compiler.h: add is_const() as a replacement of __is_constexpr()

[Martin Uecker]

Am Samstag, dem 07.12.2024 um 10:33 +0000 schrieb David Laight:
> From: Martin Uecker
> > Sent: 07 December 2024 08:40
> ...
> > I find it amazing how much time the Linux kernel community spends
> > revising code to make it work perfectly.
> > 
> > Still, I am wondering whether some of this time and effort should not
> > be targeted at C compilers and language work to make these macro
> > hacks unnecessary?
> 
> I'm probably not alone in thinking that sometimes the compiler writers
> are doing their hardest to make life hard for people writing low level code.

GCC and Clang are open-source projects just like the kernel. One can 
go there and contribute.  I am not saying that it is always easy to
find consensus and there also projects that have other requirements
than the kernel. But I started to contribute to GCC (with very limited
time) to address some of my own issues, and I found the community very
welcoming.

> 
> > I already found the original motivation for these macros very questionable.
> > Removing VLAs at the cost having imprecise worst-case bounds strikes
> > me as fundamentally misguided - at least if security is the motivation.
> 
> VLA basically cannot be allowed because of the very limited stack space.
> Even the per-frame limits aren't a real solution - they just catch the
> places that most likely to cause issues. Very deep call chains and any
> recursion (that isn't tightly bounded) can cause grief.

VLA use *less* stack than a fixed size arrays with fixed bound.
> 
> > So maybe there are other good reasons for this, e.g. bad code
> > for VLAs or risk of jumping the guard page if the attacker can somehow
> > influence its size (but for this there is -Wvla-larger-than). But even then,
> > wouldn't it be a more worthwhile and interesting investment of engineering
> > resources to improving code generation / warnings at the compiler level?
> 
> This is kernel code, any access into a stack guard page is basically
> unrecoverable for the entire system - a kernel lock/mutex could be held.
>
> With a list of (calling_fn, called_fn, stack_offset) it is possible
> calculate an accurate maximum stack usage.
> Indirect calls would need to use the (IIRC) FINE_IBT hashes to identify
> the possible functions (and I'm not sure than has an attribute for a 'seed'
> so that 'int (*)(void *)' functions can be separated into groups.
> I've not looked at whether objtool could generate the output - but is has
> to be easier for the compiler to do it.
> 
> I have done that calculation in the past (parsing a compiler listing file)
> and basically discovered the system didn't actually have enough memory
> to allocate 'safe' stacks! The max stack was pretty much always (the
> equivalent of) printf() inside an error path that never happens.
> It might be interesting to see how bad linux is (after sorting out
> how to handle recursive calls - hopefully there won't be too many
> unexpected ones.

Compiler and ISO C language support to guarantee bounded stack usage
would indeed be a very interesting feature.

> 
> > Also the fortification of strlen and co seems something which could be
> > much better solved with annotations and proper compiler support.
> 
> That might be nice, but kernel have to be buildable with relatively
> old compilers.

Yes, but it could make use of it at some point in the future (or
optionally).

> Some things might need language/ABI changes to better handle ptr+size.
> The ability to return such a pair in registers would probably be useful
> (without doing horrid games with a union and __int128).

ptr + size is something we are looking into.

You can already do quite a bit by using C99's syntax for variably modified
types. For example, you would get UBSan trap for the following OOB access:

int foo(int n, char (*buf)[n])
{
  (*buf)[n] = 1;
}

This does not require an ABI change. 

Martin