Compiler attributes: GCC cold function alignment workarounds

Contemporary versions of GCC (e.g. GCC 12.2.0) drop the alignment
specified by '-falign-functions=N' for functions marked with the
__cold__ attribute, and potentially for callees of __cold__ functions as
these may be implicitly marked as __cold__ by the compiler. LLVM appears
to respect '-falign-functions=N' in such cases.

This has been reported to GCC in bug 88345:

  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88345

... which also covers alignment being dropped when '-Os' is used, which
will be addressed in a separate patch.

Currently, use of '-falign-functions=N' is limited to
CONFIG_FUNCTION_ALIGNMENT, which is largely used for performance and/or
analysis reasons (e.g. with CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B), but
isn't necessary for correct functionality. However, this dropped
alignment isn't great for the performance and/or analysis cases.

Subsequent patches will use CONFIG_FUNCTION_ALIGNMENT as part of arm64's
ftrace implementation, which will require all instrumented functions to
be aligned to at least 8-bytes.

This patch works around the dropped alignment by avoiding the use of the
__cold__ attribute when CONFIG_FUNCTION_ALIGNMENT is non-zero, and by
specifically aligning abort(), which GCC implicitly marks as __cold__.
As the __cold macro is now dependent upon config options (which is
against the policy described at the top of compiler_attributes.h), it is
moved into compiler_types.h.

I've tested this by building and booting a kernel configured with
defconfig + CONFIG_EXPERT=y + CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B=y,
and looking for misaligned text symbols in /proc/kallsyms:

* arm64:

  Before:
    # uname -rm
    6.2.0-rc3 aarch64
    # grep ' [Tt] ' /proc/kallsyms | grep -iv '[048c]0 [Tt] ' | wc -l
    5009

  After:
    # uname -rm
    6.2.0-rc3-00001-g2a2bedf8bfa9 aarch64
    # grep ' [Tt] ' /proc/kallsyms | grep -iv '[048c]0 [Tt] ' | wc -l
    919

* x86_64:

  Before:
    # uname -rm
    6.2.0-rc3 x86_64
    # grep ' [Tt] ' /proc/kallsyms | grep -iv '[048c]0 [Tt] ' | wc -l
    11537

  After:
    # uname -rm
    6.2.0-rc3-00001-g2a2bedf8bfa9 x86_64
    # grep ' [Tt] ' /proc/kallsyms | grep -iv '[048c]0 [Tt] ' | wc -l
    2805

There's clearly a substantial reduction in the number of misaligned
symbols. From manual inspection, the remaining unaligned text labels are
a combination of ACPICA functions (due to the use of '-Os'), static call
trampolines, and non-function labels in assembly, which will be dealt
with in subsequent patches.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Florent Revest <revest@chromium.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Deacon <will@kernel.org>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20230123134603.1064407-3-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 15dc2ec80c46..c8e0375705f4 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1898,7 +1898,14 @@ bool thread_group_exited(struct pid *pid)
 }
 EXPORT_SYMBOL(thread_group_exited);
 
-__weak void abort(void)
+/*
+ * This needs to be __function_aligned as GCC implicitly makes any
+ * implementation of abort() cold and drops alignment specified by
+ * -falign-functions=N.
+ *
+ * See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88345#c11
+ */
+__weak __function_aligned void abort(void)
 {
 	BUG();