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Date: Thu, 27 Jan 2022 12:20:32 +0000 From: Mark Rutland <mark.rutland@....com> To: Ard Biesheuvel <ardb@...nel.org> Cc: Yinan Liu <yinan@...ux.alibaba.com>, Steven Rostedt <rostedt@...dmis.org>, "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" <linuxppc-dev@...ts.ozlabs.org>, Sachin Sant <sachinp@...ux.ibm.com>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Kees Cook <keescook@...omium.org> Subject: Re: [powerpc] ftrace warning kernel/trace/ftrace.c:2068 with code-patching selftests On Thu, Jan 27, 2022 at 01:03:34PM +0100, Ard Biesheuvel wrote: > On Thu, 27 Jan 2022 at 12:47, Mark Rutland <mark.rutland@....com> wrote: > > > > [adding LKML so this is easier for others to find] > > > > If anyone wants to follow the thread from the start, it's at: > > > > https://lore.kernel.org/linuxppc-dev/944D10DA-8200-4BA9-8D0A-3BED9AA99F82@linux.ibm.com/ > > > > Ard, I was under the impression that the 32-bit arm kernel was (virtually) > > relocatable, but I couldn't spot where, and suspect I'm mistaken. Do you know > > whether it currently does any boot-time dynamic relocation? > > No, it does not. Thanks for comfirming! So 32-bit arm should be able to opt into the build-time sort as-is. > > Steve asked for a bit more detail on IRC, so the below is an attempt to explain > > what's actually going on here. > > > > The short answer is that relocatable kernels (e.g. those with KASLR support) > > need to handle the kernel being loaded at (somewhat) arbitrary virtual > > addresses. Even where code can be position-independent, any pointers in the > > kernel image (e.g. the contents of the mcount_loc table) need to be updated to > > account for the specific VA the kernel was loaded at -- arch code does this > > early at boot time by applying dynamic (ELF) relocations. > > 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. Thanks, Mark.
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