[PATCH v6 02/14] aarch64: configure test for BTI support

[Szabolcs Nagy]

The 07/06/2020 15:12, Adhemerval Zanella wrote:
> 
> 
> On 06/07/2020 15:07, Szabolcs Nagy wrote:
> > The 07/06/2020 11:11, Adhemerval Zanella via Libc-alpha wrote:
> >>
> >>
> >> On 01/07/2020 11:38, Szabolcs Nagy wrote:
> >>> Check BTI support in the compiler and linker.  The check also
> >>> requires READELF that understands the BTI GNU property note.
> >>> It is expected to succeed with gcc >=gcc-9 configured with
> >>> --enable-standard-branch-protection and binutils >=binutils-2.33.
> >>>
> >>> Note: passing -mbranch-protection=bti in CFLAGS when building glibc
> >>> may not be enough to get a glibc that supports BTI because crtbegin*
> >>> and crtend* provided by the compiler needs to be BTI compatible too.
> >>
> >> If I read correctly this scenario should be covered by the configure
> >> test, right?
> > 
> > it is not covered because the config checks use -nostdlib
> > and -nostartfiles (otherwise objects from the libc that
> > the toolchain uses would leak into the checks).
> > 
> > so if gcc startfiles are not bti compatible then configure
> > succeeds (but later on there will be link failures so the
> > build fails).
> 
> Right, would be hard to try to fail early on configure for this
> case?

i don't know how to reliably link with gcc start files
but not with libc start files.

explicit link wiht crtbeginS.o may not be right, the
exact name depends on the compiler driver.

so i would have to provide dummy libc start files to be
able to do the check, but that can go bad in many ways
so the check may not be reliable.