Assembly: enable CET

[sneves]

Addresses #95.

With our current dispatcher, both on C and Rust, having endbr64 at the top of the functions is rather pointless since these will never be at the end of an indirect call. But since this is a glorified nop, might as well.

[oconnor663]

I have no idea what this means. Can I leave it to you to hit the merge button whenever you think it's ready? :)

[xnox]

Looks good to me! The test C binary is marked as CET enabled.

Please merge.

[xnox]

I have no idea what this means. Can I leave it to you to hit the merge button whenever you think it's ready? :)

I find this presentation the best explanation about the CET technology https://www.linuxplumbersconf.org/event/2/contributions/147/attachments/72/83/CET-LPC-2018.pdf

[xnox]

Maybe @hjl-tools could review this too. This adds CET support to BLAKE3 asm.

[hjl-tools]

Since the assembly codes are in .S, you should do

#if defined(__ELF__) && defined(__CET__)
# include <cet.h>
#else
# define _CET_ENDBR
#endif

and use _CET_ENDBR everywhere ENDBR is needed. Since -fcf-protection needs binutils with CET support,
it should work.

[sneves]

The reason I did not just include the cet.h header is that it appears to be GCC-specific; building with Clang would fail, even though it supports -fcf-protection=full.

[hjl-tools]

The reason I did not just include the cet.h header is that it appears to be GCC-specific; building with Clang would fail, even though it supports -fcf-protection=full.

LLVM doesn't working CET support before LLVM 11 which also includes <cet.h>:

https://bugs.llvm.org/show_bug.cgi?id=45484

Most of CET fixes have been backported to LLVM 10.x, except for <cet.h>. You can do

#if defined(__ELF__) && defined(__CET__) && __has_include(<cet.h>)
# include <cet.h>
#else
...
#endif

[sneves]

Most of CET fixes have been backported to LLVM 10.x, except for <cet.h>. You can do

#if defined(__ELF__) && defined(__CET__) && __has_include(<cet.h>)
# include <cet.h>
#else
...
#endif

Goes in as suggested.

[xnox]

Thank you @hjl-tools for your input! Much appreciated!