Messages in this thread | | | Date | Thu, 27 Jan 2022 12:59:31 +0000 | From | Mark Rutland <> | Subject | Re: [powerpc] ftrace warning kernel/trace/ftrace.c:2068 with code-patching selftests |
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On Thu, Jan 27, 2022 at 01:22:17PM +0100, Ard Biesheuvel wrote: > On Thu, 27 Jan 2022 at 13:20, Mark Rutland <mark.rutland@arm.com> wrote: > > On Thu, Jan 27, 2022 at 01:03:34PM +0100, Ard Biesheuvel wrote: > > > > > These architectures use place-relative extables for the same reason: > > > place relative references are resolved at build time rather than at > > > runtime during relocation, making a build time sort feasible. > > > > > > arch/alpha/include/asm/extable.h:#define ARCH_HAS_RELATIVE_EXTABLE > > > arch/arm64/include/asm/extable.h:#define ARCH_HAS_RELATIVE_EXTABLE > > > arch/ia64/include/asm/extable.h:#define ARCH_HAS_RELATIVE_EXTABLE > > > arch/parisc/include/asm/uaccess.h:#define ARCH_HAS_RELATIVE_EXTABLE > > > arch/powerpc/include/asm/extable.h:#define ARCH_HAS_RELATIVE_EXTABLE > > > arch/riscv/include/asm/extable.h:#define ARCH_HAS_RELATIVE_EXTABLE > > > arch/s390/include/asm/extable.h:#define ARCH_HAS_RELATIVE_EXTABLE > > > arch/x86/include/asm/extable.h:#define ARCH_HAS_RELATIVE_EXTABLE > > > > > > Note that the swap routine becomes something like the below, given > > > that the relative references need to be fixed up after the entry > > > changes place in the sorted list. > > > > > > static void swap_ex(void *a, void *b, int size) > > > { > > > struct exception_table_entry *x = a, *y = b, tmp; > > > int delta = b - a; > > > > > > tmp = *x; > > > x->insn = y->insn + delta; > > > y->insn = tmp.insn - delta; > > > ... > > > } > > > > > > As a bonus, the resulting footprint of the table in the image is > > > reduced by 8x, given that every 8 byte pointer has an accompanying 24 > > > byte RELA record, so we go from 32 bytes to 4 bytes for every call to > > > __gnu_mcount_mc. > > > > Absolutely -- it'd be great if we could do that for the callsite locations; the > > difficulty is that the entries are generated by the compiler itself, so we'd > > either need some build/link time processing to convert each absolute 64-bit > > value to a relative 32-bit offset, or new compiler options to generate those as > > relative offsets from the outset. > > Don't we use scripts/recordmcount.pl for that?
Not quite -- we could adjust it to do so, but today it doesn't consider existing mcount_loc entries, and just generates new ones where the compiler has generated calls to mcount, which it finds by scanning the instructions in the binary. Today it is not used:
* On arm64 when we default to using `-fpatchable-function-entry=N`. That makes the compiler insert 2 NOPs in the function prologue, and log the location of that NOP sled to a section called. `__patchable_function_entries`.
We need the compiler to do that since we can't reliably identify 2 NOPs in a function prologue as being intended to be a patch site, as e.g. there could be notrace functions where the compiler had to insert NOPs for alignment of a subsequent brnach or similar.
* On architectures with `-nop-mcount`. On these, it's necessary to use `-mrecord-mcount` to have the compiler log the patch-site, for the same reason as with `-fpatchable-function-entry`.
* On architectures with `-mrecord-mcount` generally, since this avoids the overhead of scanning each object.
* On x86 when objtool is used.
Thanks, Mark.
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