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Date: Thu, 25 Mar 2021 17:03:08 -0700 From: Peter Collingbourne <pcc@...gle.com> To: Sami Tolvanen <samitolvanen@...gle.com> Cc: Mark Rutland <mark.rutland@....com>, Kees Cook <keescook@...omium.org>, Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, Masahiro Yamada <masahiroy@...nel.org>, Will Deacon <will@...nel.org>, Jessica Yu <jeyu@...nel.org>, Arnd Bergmann <arnd@...db.de>, Tejun Heo <tj@...nel.org>, "Paul E. McKenney" <paulmck@...nel.org>, Christoph Hellwig <hch@...radead.org>, Peter Zijlstra <peterz@...radead.org>, bpf <bpf@...r.kernel.org>, linux-hardening@...r.kernel.org, linux-arch <linux-arch@...r.kernel.org>, linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>, linux-kbuild <linux-kbuild@...r.kernel.org>, PCI <linux-pci@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH v3 12/17] arm64: implement __va_function On Thu, Mar 25, 2021 at 4:28 PM Sami Tolvanen <samitolvanen@...gle.com> wrote: > > On Thu, Mar 25, 2021 at 3:38 AM Mark Rutland <mark.rutland@....com> 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@...gle.com> > > > Reviewed-by: Kees Cook <keescook@...omium.org> > > > > 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
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