On Fri, Mar 19, 2021 at 7:55 PM Greg Kroah-Hartman
[off-list ref] wrote:

On Fri, Mar 19, 2021 at 12:20:22PM +0100, Alexandre Chartre wrote:

quoted

On 3/19/21 11:39 AM, Greg Kroah-Hartman wrote:

quoted

On Fri, Mar 19, 2021 at 07:54:15AM +0800, Nicolas Boichat wrote:

quoted

From: Thomas Gleixner <redacted>

commit 6553896666433e7efec589838b400a2a652b3ffa upstream.

Some code pathes, especially the low level entry code, must be protected
against instrumentation for various reasons:

  - Low level entry code can be a fragile beast, especially on x86.

  - With NO_HZ_FULL RCU state needs to be established before using it.

Having a dedicated section for such code allows to validate with tooling
that no unsafe functions are invoked.

Add the .noinstr.text section and the noinstr attribute to mark
functions. noinstr implies notrace. Kprobes will gain a section check
later.

Provide also a set of markers: instrumentation_begin()/end()

These are used to mark code inside a noinstr function which calls
into regular instrumentable text section as safe.

The instrumentation markers are only active when CONFIG_DEBUG_ENTRY is
enabled as the end marker emits a NOP to prevent the compiler from merging
the annotation points. This means the objtool verification requires a
kernel compiled with this option.

Signed-off-by: Thomas Gleixner <redacted>
Reviewed-by: Alexandre Chartre <redacted>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200505134100.075416272@linutronix.de

[Nicolas: context conflicts in:
  arch/powerpc/kernel/vmlinux.lds.S
  include/asm-generic/vmlinux.lds.h
  include/linux/compiler.h
  include/linux/compiler_types.h]
Signed-off-by: Nicolas Boichat <redacted>

Did you build this on x86?

I get the following build error:

ld:./arch/x86/kernel/vmlinux.lds:20: syntax error

And that line looks like:

  . = ALIGN(8); *(.text.hot .text.hot.*) *(.text .text.fixup) *(.text.unlikely .text.unlikely.*) *(.text.unknown .text.unknown.*) . = ALIGN(8); __noinstr_text_start = .; *(.__attribute__((noinline)) __attribute__((no_instrument_function)) __attribute((__section__(".noinstr.text"))).text) __noinstr_text_end = .; *(.text..refcount) *(.ref.text) *(.meminit.text*) *(.memexit.text*)

In the NOINSTR_TEXT macro, noinstr is expanded with the value of the noinstr
macro from linux/compiler_types.h while it shouldn't.

The problem is possibly that the noinstr macro is defined for assembly. Make
sure that the macro is not defined for assembly e.g.:

#ifndef __ASSEMBLY__

/* Section for code which can't be instrumented at all */
#define noinstr                                                               \
      noinline notrace __attribute((__section__(".noinstr.text")))

#endif

This implies that the backport is incorrect, so I'll wait for an updated
version...

Yep, sorry about that. I did test on ARM64 only and these patches
happily went through our Chrome OS CQ (we don't have gcc coverage
though).

Guenter has a fixup here with explanation:
https://crrev.com/c/2776332, I'll look carefully and resubmit.

Thanks,

thanks,

greg k-h