On Thu, Mar 25, 2021 at 4:28 PM Sami Tolvanen <samitolvanen@xxxxxxxxxx> wrote: > > On Thu, Mar 25, 2021 at 3:38 AM Mark Rutland <mark.rutland@xxxxxxx> wrote: > > > > On Tue, Mar 23, 2021 at 01:39:41PM -0700, Sami Tolvanen wrote: > > > With CONFIG_CFI_CLANG, the compiler replaces function addresses in > > > instrumented C code with jump table addresses. This change implements > > > the __va_function() macro, which returns the actual function address > > > instead. > > > > > > Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx> > > > Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> > > > > Is there really no attribute or builtin that can be used to do this > > without assembly? > > I don't think the compiler currently offers anything that could help > us here. Peter, can you think of another way to avoid the function > address to jump table address conversion with > -fno-sanitize-cfi-canonical-jump-tables? No, the assembly seems like the best way at the moment. > > IIUC from other patches the symbol tables will contain the "real" > > non-cfi entry points (unless we explciitly asked to make the jump table > > address canonical), so AFAICT here the compiler should have all the > > necessary information to generate either the CFI or non-CFI entry point > > addresses, even if it doesn't expose an interface for that today. > > > > It'd be a lot nicer if we could get the compiler to do this for us. > > I agree, that would be quite useful in the kernel. Maybe. The kernel's requirements seem quite specialized here though. A normal C or C++ program has little need for the actual entry point, so if you need it you are probably doing something low level enough to require assembly anyway. Peter
