On Wed, Oct 30, 2019 at 04:03:02PM +0100, Torsten Duwe wrote:
> On Tue, Oct 29, 2019 at 04:58:26PM +0000, Mark Rutland wrote:
> > When using patchable-function-entry, the compiler will record the
> > callsites into a section named "__patchable_function_entries" rather
> > than "__mcount_loc". Let's abstract this difference behind a new
> > FTRACE_CALLSITE_SECTION, so that architectures don't have to handle this
> > explicitly (e.g. with custom module linker scripts).
> >
> > As parisc currently handles this explicitly, it is fixed up accordingly,
> > with its custom linker script removed. Since FTRACE_CALLSITE_SECTION is
> > only defined when DYNAMIC_FTRACE is selected, the parisc module loading
> > code is updated to only use the definition in that case. When
> > DYNAMIC_FTRACE is not selected, modules shouldn't have this section, so
> > this removes some redundant work in that case.
> >
> > I built parisc generic-{32,64}bit_defconfig with DYNAMIC_FTRACE enabled,
> > and verified that the section made it into the .ko files for modules.
>
> This is because of remaining #ifdeffery in include/asm-generic/vmlinux.lds.h:
>
> #ifdef CC_USING_PATCHABLE_FUNCTION_ENTRY
> #define MCOUNT_REC() . = ALIGN(8); \
> __start_mcount_loc = .; \
> KEEP(*(__patchable_function_entries)) \
> __stop_mcount_loc = .;
> #else
> #define MCOUNT_REC() . = ALIGN(8); \
> __start_mcount_loc = .; \
> KEEP(*(__mcount_loc)) \
> __stop_mcount_loc = .;
> #endif
For modules we use a combination of scripts/module-common.lds and an
architecture's own module.lds, not vmlinux.lds.h. So I don't think the above is
relevant for modules.
For modules the kernel's ELF loader looks for the ELF ection, not the
__{start,stop}_mcount_loc symbols that we use for the main kernel image.
FWIW, when building a module, I see the following linker operations:
| [mark@blommer:~/src/linux]% toolchain korg gcc-8.1.0-nolibc make V=1 ARCH=arm64 CROSS_COMPILE=aarch64-linux- arch/arm64/crypto/sha512-ce.ko | grep aarch64-linux-ld
| aarch64-linux-ld -EL -maarch64elf -r -o arch/arm64/crypto/sha512-ce.o arch/arm64/crypto/sha512-ce-glue.o arch/arm64/crypto/sha512-ce-core.o
| aarch64-linux-ld -r -EL -maarch64elf --build-id -T ./scripts/module-common.lds -T ./arch/arm64/kernel/module.lds -o arch/arm64/crypto/sha512-ce.ko arch/arm64/crypto/sha512-ce.o arch/arm64/crypto/sha512-ce.mod.o; true
> Maybe you want to tackle that as well? I suggest to have at least one
> FTRACE_CALLSITE_SECTION definition without double quotes. Alternatively, my
> earlier solution just kept both sections, in case either one or both are
> present.
>
> KEEP(*(__patchable_function_entries)) \
> KEEP(*(__mcount_loc)) \
I agree that the CC_USING_PATCHABLE_FUNCTION_ENTRY ifdeffery could be
simplified, and that it would be nice to consistently use
FTRACE_CALLSITE_SECTION if we can. However, the generic linker script doesn't
include anything, and I don't see a good location for that to live.
What I could do is add an explicit comment:
/*
* The ftrace call sites are logged to a section whose name depends on the
* compiler option used. A given kernel image will only use one, AKA
* FTRACE_CALLSITE_SECTION. We capture all of them here to avoid header
* dependencies.
*/
#define MCOUNT_REC() \
. = ALIGN(8); \
__start_mcount_loc = .; \
KEEP(*(__patchable_function_entries)) \
KEEP(*(__mcount_loc)) \
__stop_mcount_loc = .;
... which should make the dependency clear. Does that sound good to you?
Thanks,
Mark.
