On Thu, May 04, 2023 at 07:43:20PM +0200, Ard Biesheuvel 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> > 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 = .); \ Perhaps add comments here and in kernel/kallsyms_internal.h to correlate this list and the externs? "This list is needed because ..., see the externs in kallsyms_internal.h." or similar? Otherwise, yeah, looks good. Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> -- Kees Cook
