| Message ID | 20240522114755.318238-3-masahiroy@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | kbuild: remove PROVIDE() and refactor vmlinux_link steps | expand |
On Wed, 22 May 2024 at 13:48, Masahiro Yamada <masahiroy@kernel.org> wrote: > > This reimplements commit 951bcae6c5a0 ("kallsyms: Avoid weak references > for kallsyms symbols"). > > I am not a big fan of PROVIDE() because it always satisfies the linker > even in situations that should result in a link error. In other words, > it can potentially shift a compile-time error into a run-time error. > I don't disagree. However, I did realize that, in this particular case, we could at least make the preliminary symbol definitions conditional on CONFIG_KALLSYMS rather than always providing them. This approach is also fine with me, though. > Duplicating kallsyms_* in vmlinux.lds.h also reduces maintainability. > > As an alternative solution, this commit prepends one more kallsyms step. > > KSYMS .tmp_vmlinux.kallsyms0.S # added > AS .tmp_vmlinux.kallsyms0.o # added > LD .tmp_vmlinux.btf > BTF .btf.vmlinux.bin.o > LD .tmp_vmlinux.kallsyms1 > NM .tmp_vmlinux.kallsyms1.syms > KSYMS .tmp_vmlinux.kallsyms1.S > AS .tmp_vmlinux.kallsyms1.o > LD .tmp_vmlinux.kallsyms2 > NM .tmp_vmlinux.kallsyms2.syms > KSYMS .tmp_vmlinux.kallsyms2.S > AS .tmp_vmlinux.kallsyms2.o > LD vmlinux > > Step 0 takes /dev/null as input, and generates .tmp_vmlinux.kallsyms0.o, > which has a valid kallsyms format with the empty symbol list, and can be > linked to vmlinux. Since it is really small, the added compile-time cost > is negligible. > OK, so the number of linker invocations is the same, right? The difference is that the kallsyms symbol references are satisfied by a dummy object? That seems reasonable to me. For the series, Acked-by: Ard Biesheuvel <ardb@kernel.org>
On Thu, May 23, 2024 at 6:32 PM Ard Biesheuvel <ardb@kernel.org> wrote: > > On Wed, 22 May 2024 at 13:48, Masahiro Yamada <masahiroy@kernel.org> wrote: > > > > This reimplements commit 951bcae6c5a0 ("kallsyms: Avoid weak references > > for kallsyms symbols"). > > > > I am not a big fan of PROVIDE() because it always satisfies the linker > > even in situations that should result in a link error. In other words, > > it can potentially shift a compile-time error into a run-time error. > > > > I don't disagree. However, I did realize that, in this particular > case, we could at least make the preliminary symbol definitions > conditional on CONFIG_KALLSYMS rather than always providing them. Fair enough. I am fine with dropping this statement. > > This approach is also fine with me, though. > > > > Duplicating kallsyms_* in vmlinux.lds.h also reduces maintainability. > > > > As an alternative solution, this commit prepends one more kallsyms step. > > > > KSYMS .tmp_vmlinux.kallsyms0.S # added > > AS .tmp_vmlinux.kallsyms0.o # added > > LD .tmp_vmlinux.btf > > BTF .btf.vmlinux.bin.o > > LD .tmp_vmlinux.kallsyms1 > > NM .tmp_vmlinux.kallsyms1.syms > > KSYMS .tmp_vmlinux.kallsyms1.S > > AS .tmp_vmlinux.kallsyms1.o > > LD .tmp_vmlinux.kallsyms2 > > NM .tmp_vmlinux.kallsyms2.syms > > KSYMS .tmp_vmlinux.kallsyms2.S > > AS .tmp_vmlinux.kallsyms2.o > > LD vmlinux > > > > Step 0 takes /dev/null as input, and generates .tmp_vmlinux.kallsyms0.o, > > which has a valid kallsyms format with the empty symbol list, and can be > > linked to vmlinux. Since it is really small, the added compile-time cost > > is negligible. > > > > OK, so the number of linker invocations is the same, right? The > difference is that the kallsyms symbol references are satisfied by a > dummy object? Correct. In 3/3, I even reduce the number of link steps when both CONFIG_DEBUG_INFO_BTF and CONFIG_KALLSYMS are enabled. > > That seems reasonable to me. > > For the series, > > Acked-by: Ard Biesheuvel <ardb@kernel.org> >
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h index 5703526d6ebf..62b4cb0462e6 100644 --- a/include/asm-generic/vmlinux.lds.h +++ b/include/asm-generic/vmlinux.lds.h @@ -451,30 +451,11 @@ #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_internal.h b/kernel/kallsyms_internal.h index 85480274fc8f..925f2ab22639 100644 --- a/kernel/kallsyms_internal.h +++ b/kernel/kallsyms_internal.h @@ -4,11 +4,6 @@ #include <linux/types.h> -/* - * 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[]; extern const int kallsyms_offsets[]; extern const u8 kallsyms_names[]; diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c index 47978efe4797..fa53b5eef553 100644 --- a/scripts/kallsyms.c +++ b/scripts/kallsyms.c @@ -259,12 +259,6 @@ static void shrink_table(void) } } table_cnt = pos; - - /* When valid symbol is not registered, exit to error */ - if (!table_cnt) { - fprintf(stderr, "No valid symbol.\n"); - exit(1); - } } static void read_map(const char *in) diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh index b16967d33f1c..fe7db9a265ca 100755 --- a/scripts/link-vmlinux.sh +++ b/scripts/link-vmlinux.sh @@ -225,6 +225,11 @@ ${MAKE} -f "${srctree}/scripts/Makefile.build" obj=init init/version-timestamp.o kallsymso= btf_vmlinux_bin_o= +if is_enabled CONFIG_KALLSYMS; then + # kallsyms step 0 + kallsyms /dev/null .tmp_vmlinux.kallsyms0 +fi + if is_enabled CONFIG_DEBUG_INFO_BTF; then if ! gen_btf .tmp_vmlinux.btf .btf.vmlinux.bin.o ; then echo >&2 "Failed to generate BTF for vmlinux" @@ -237,9 +242,10 @@ if is_enabled CONFIG_KALLSYMS; then # kallsyms support # Generate section listing all symbols and add it into vmlinux - # It's a three step process: + # It's a four step process: + # 0) Generate a dummy __kallsyms with empty symbol list. # 1) Link .tmp_vmlinux.kallsyms1 so it has all symbols and sections, - # but __kallsyms is empty. + # with a dummy __kallsyms. # Running kallsyms on that gives us .tmp_kallsyms1.o with # the right size # 2) Link .tmp_vmlinux.kallsyms2 so it now has a __kallsyms section of
This reimplements commit 951bcae6c5a0 ("kallsyms: Avoid weak references for kallsyms symbols"). I am not a big fan of PROVIDE() because it always satisfies the linker even in situations that should result in a link error. In other words, it can potentially shift a compile-time error into a run-time error. Duplicating kallsyms_* in vmlinux.lds.h also reduces maintainability. As an alternative solution, this commit prepends one more kallsyms step. KSYMS .tmp_vmlinux.kallsyms0.S # added AS .tmp_vmlinux.kallsyms0.o # added LD .tmp_vmlinux.btf BTF .btf.vmlinux.bin.o LD .tmp_vmlinux.kallsyms1 NM .tmp_vmlinux.kallsyms1.syms KSYMS .tmp_vmlinux.kallsyms1.S AS .tmp_vmlinux.kallsyms1.o LD .tmp_vmlinux.kallsyms2 NM .tmp_vmlinux.kallsyms2.syms KSYMS .tmp_vmlinux.kallsyms2.S AS .tmp_vmlinux.kallsyms2.o LD vmlinux Step 0 takes /dev/null as input, and generates .tmp_vmlinux.kallsyms0.o, which has a valid kallsyms format with the empty symbol list, and can be linked to vmlinux. Since it is really small, the added compile-time cost is negligible. Signed-off-by: Masahiro Yamada <masahiroy@kernel.org> --- include/asm-generic/vmlinux.lds.h | 19 ------------------- kernel/kallsyms_internal.h | 5 ----- scripts/kallsyms.c | 6 ------ scripts/link-vmlinux.sh | 10 ++++++++-- 4 files changed, 8 insertions(+), 32 deletions(-)