On Tue, Apr 16, 2024 at 1:20 AM Ard Biesheuvel <ardb+git@xxxxxxxxxx> wrote: > > From: Ard Biesheuvel <ardb@xxxxxxxxxx> > > 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. > > Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> # Boot > Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> > Acked-by: Arnd Bergmann <arnd@xxxxxxxx> > Link: https://lkml.kernel.org/r/20230504174320.3930345-1-ardb%40kernel.org > Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx> > --- I dropped v5, and picked up this one. Thanks. > include/asm-generic/vmlinux.lds.h | 19 +++++++++++++ > kernel/kallsyms.c | 6 ---- > kernel/kallsyms_internal.h | 30 ++++++++------------ > 3 files changed, 31 insertions(+), 24 deletions(-) > > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h > index f7749d0f2562..e8449be62058 100644 > --- a/include/asm-generic/vmlinux.lds.h > +++ b/include/asm-generic/vmlinux.lds.h > @@ -448,11 +448,30 @@ > #endif > #endif > > +/* > + * Some symbol definitions will not exist yet during the first pass of the > + * link, but are guaranteed to exist in the final link. Provide preliminary > + * definitions that will be superseded in the final link to avoid having to > + * rely on weak external linkage, which requires a GOT when used in position > + * independent code. > + */ > +#define PRELIMINARY_SYMBOL_DEFINITIONS \ > + 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 = .); > + > /* > * Read only Data > */ > #define RO_DATA(align) \ > . = ALIGN((align)); \ > + PRELIMINARY_SYMBOL_DEFINITIONS \ > .rodata : AT(ADDR(.rodata) - LOAD_OFFSET) { \ > __start_rodata = .; \ > *(.rodata) *(.rodata.*) \ > diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c > index 18edd57b5fe8..22ea19a36e6e 100644 > --- a/kernel/kallsyms.c > +++ b/kernel/kallsyms.c > @@ -325,12 +325,6 @@ 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); > - > /* 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 27fabdcc40f5..85480274fc8f 100644 > --- a/kernel/kallsyms_internal.h > +++ b/kernel/kallsyms_internal.h > @@ -5,27 +5,21 @@ > #include <linux/types.h> > > /* > - * These will be re-linked against their real values > - * during the second link stage. > + * These will be re-linked against their real values during the second link > + * stage. Preliminary values must be provided in the linker script using the > + * PROVIDE() directive so that the first link stage can complete successfully. > */ > -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_ > -- > 2.44.0.683.g7961c838ac-goog > > -- Best Regards Masahiro Yamada
