Date: Thu, 25 Jun 2020 09:15:03 -0700 From: Sami Tolvanen <samitolvanen@...gle.com> To: Peter Zijlstra <peterz@...radead.org> Cc: Steven Rostedt <rostedt@...dmis.org>, Masahiro Yamada <masahiroy@...nel.org>, Will Deacon <will@...nel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Paul E. McKenney" <paulmck@...nel.org>, Kees Cook <keescook@...omium.org>, Nick Desaulniers <ndesaulniers@...gle.com>, clang-built-linux@...glegroups.com, kernel-hardening@...ts.openwall.com, linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org, x86@...nel.org Subject: Re: [PATCH 04/22] kbuild: lto: fix recordmcount On Thu, Jun 25, 2020 at 09:45:30AM +0200, Peter Zijlstra wrote: > On Wed, Jun 24, 2020 at 02:45:30PM -0700, Sami Tolvanen wrote: > > On Wed, Jun 24, 2020 at 11:27:37PM +0200, Peter Zijlstra wrote: > > > On Wed, Jun 24, 2020 at 01:31:42PM -0700, Sami Tolvanen wrote: > > > > With LTO, LLVM bitcode won't be compiled into native code until > > > > modpost_link. This change postpones calls to recordmcount until after > > > > this step. > > > > > > > > In order to exclude specific functions from inspection, we add a new > > > > code section .text..nomcount, which we tell recordmcount to ignore, and > > > > a __nomcount attribute for moving functions to this section. > > > > > > I'm confused, you only add this to functions in ftrace itself, which is > > > compiled with: > > > > > > KBUILD_CFLAGS = $(subst $(CC_FLAGS_FTRACE),,$(ORIG_CFLAGS)) > > > > > > and so should not have mcount/fentry sites anyway. So what's the point > > > of ignoring them further? > > > > > > This Changelog does not explain. > > > > Normally, recordmcount ignores each ftrace.o file, but since we are > > running it on vmlinux.o, we need another way to stop it from looking > > at references to mcount/fentry that are not calls. Here's a comment > > from recordmcount.c: > > > > /* > > * The file kernel/trace/ftrace.o references the mcount > > * function but does not call it. Since ftrace.o should > > * not be traced anyway, we just skip it. > > */ > > > > But I agree, the commit message could use more defails. Also +Steven > > for thoughts about this approach. > > Ah, is thi because recordmcount isn't smart enough to know the > difference between "CALL $mcount" and any other RELA that has mcount? Yes. > At least for x86_64 I can do a really quick take for a recordmcount pass > in objtool, but I suppose you also need this for ARM64 ? Sure, sounds good. arm64 uses -fpatchable-function-entry with clang, so we don't need recordmcount there. Sami
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