On Thu, May 4, 2023 at 10:43 AM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote: > > kallsyms is a directory of all the symbols in the vmlinux binary, and so > creating it is somewhat of a chicken-and-egg problem, as its non-zero > size affects the layout of the binary, and therefore the values of the > symbols. > > For this reason, the kernel is linked more than once, and the first pass > does not include any kallsyms data at all. For the linker to accept > this, the symbol declarations describing the kallsyms metadata are > emitted as having weak linkage, so they can remain unsatisfied. During > the subsequent passes, the weak references are satisfied by the kallsyms > metadata that was constructed based on information gathered from the > preceding passes. > > Weak references lead to somewhat worse codegen, because taking their > address may need to produce NULL (if the reference was unsatisfied), and > this is not usually supported by RIP or PC relative symbol references. > > Given that these references are ultimately always satisfied in the final > link, let's drop the weak annotation, and instead, provide fallback > definitions in the linker script that are only emitted if an unsatisfied > reference exists. > > While at it, drop the FRV specific annotation that these symbols reside > in .rodata - FRV is long gone. > > Cc: Arnd Bergmann <arnd@xxxxxxxx> > Cc: Fangrui Song <maskray@xxxxxxxxxx> > Cc: Nathan Chancellor <nathan@xxxxxxxxxx> > Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> Thanks for the patch. I did some quick boot tests of this with: - x86 defconfig + CONFIG_KALLSYMS_SELFTEST=y - x86 defconfig + CONFIG_KALLSYMS_SELFTEST=y + CONFIG_LTO_CLANG_THIN=y - arm64 defconfig + CONFIG_KALLSYMS_SELFTEST=y Curiously, I only see: [ 1.002200] kallsyms_selftest: start in the output (when grepping for kallsyms_selftest as instructed by the help text for KALLSYMS_SELFTEST in init/Kconfig). But that happens regardless of this patch. I did not test backtraces or live patching (seems like kallsyms is related to those reading through the help texts in init/Kconfig), or measure for binary changes. Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> # Boot Based on my interpretation of https://sourceware.org/binutils/docs/ld/PROVIDE.html, this LGTM. Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > Cc: Kees Cook <keescook@xxxxxxxxxxxx> > Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx> > --- > include/asm-generic/vmlinux.lds.h | 9 +++++++ > kernel/kallsyms.c | 6 ----- > kernel/kallsyms_internal.h | 25 +++++++------------- > 3 files changed, 18 insertions(+), 22 deletions(-) > > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h > index d1f57e4868ed341d..dd42c0fcad2b519f 100644 > --- a/include/asm-generic/vmlinux.lds.h > +++ b/include/asm-generic/vmlinux.lds.h > @@ -460,6 +460,15 @@ > */ > #define RO_DATA(align) \ > . = ALIGN((align)); \ > + PROVIDE(kallsyms_addresses = .); \ > + PROVIDE(kallsyms_offsets = .); \ > + PROVIDE(kallsyms_names = .); \ > + PROVIDE(kallsyms_num_syms = .); \ > + PROVIDE(kallsyms_relative_base = .); \ > + PROVIDE(kallsyms_token_table = .); \ > + PROVIDE(kallsyms_token_index = .); \ > + PROVIDE(kallsyms_markers = .); \ > + PROVIDE(kallsyms_seqs_of_names = .); \ > .rodata : AT(ADDR(.rodata) - LOAD_OFFSET) { \ > __start_rodata = .; \ > *(.rodata) *(.rodata.*) \ > diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c > index 77747391f49b66cb..5b16009ee53aa05b 100644 > --- a/kernel/kallsyms.c > +++ b/kernel/kallsyms.c > @@ -331,12 +331,6 @@ static unsigned long get_symbol_pos(unsigned long addr, > unsigned long symbol_start = 0, symbol_end = 0; > unsigned long i, low, high, mid; > > - /* This kernel should never had been booted. */ > - if (!IS_ENABLED(CONFIG_KALLSYMS_BASE_RELATIVE)) > - BUG_ON(!kallsyms_addresses); > - else > - BUG_ON(!kallsyms_offsets); Even previously with weak definitions, wouldn't these values always be true? > - > /* Do a binary search on the sorted kallsyms_addresses array. */ > low = 0; > high = kallsyms_num_syms; > diff --git a/kernel/kallsyms_internal.h b/kernel/kallsyms_internal.h > index 27fabdcc40f57931..cf4124dbcc5b6d0e 100644 > --- a/kernel/kallsyms_internal.h > +++ b/kernel/kallsyms_internal.h > @@ -8,24 +8,17 @@ > * These will be re-linked against their real values > * during the second link stage. > */ > -extern const unsigned long kallsyms_addresses[] __weak; > -extern const int kallsyms_offsets[] __weak; > -extern const u8 kallsyms_names[] __weak; > +extern const unsigned long kallsyms_addresses[]; > +extern const int kallsyms_offsets[]; > +extern const u8 kallsyms_names[]; > > -/* > - * Tell the compiler that the count isn't in the small data section if the arch > - * has one (eg: FRV). > - */ > -extern const unsigned int kallsyms_num_syms > -__section(".rodata") __attribute__((weak)); > - > -extern const unsigned long kallsyms_relative_base > -__section(".rodata") __attribute__((weak)); > +extern const unsigned int kallsyms_num_syms; > +extern const unsigned long kallsyms_relative_base; > > -extern const char kallsyms_token_table[] __weak; > -extern const u16 kallsyms_token_index[] __weak; > +extern const char kallsyms_token_table[]; > +extern const u16 kallsyms_token_index[]; > > -extern const unsigned int kallsyms_markers[] __weak; > -extern const u8 kallsyms_seqs_of_names[] __weak; > +extern const unsigned int kallsyms_markers[]; > +extern const u8 kallsyms_seqs_of_names[]; > > #endif // LINUX_KALLSYMS_INTERNAL_H_ > -- > 2.39.2 > -- Thanks, ~Nick Desaulniers
