Re: [PATCH] Arm: do a 4th linking pass if necessary

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 5/20/26 1:53 PM, Jan Beulich wrote:
> The .rodata growth between 1st and 2nd linking passes may affect the
> number of stubs the linker wants to insert, which in turn affects the
> number of symbols. If symbol table sizes change after the 2nd linking
> pass, insert another one before the final one. (As a comment in Linux puts
> it, "In theory it's possible this results in even more stubs, but
> unlikely.")
>
> To use the $(compare-symbol-tables) macro in a shell "if", it needs
> slightly adjusting (and then wrapping to be run in a sub-shell).
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> There's imo no good Fixes: tag, since (aiui) stubs (veneers) may also be
> inserted for reasons other than errata workarounds.
>
> For context: The Cortex A53 erratum 843419 workaround in GNU ld comes in
> two flavors: In the general case a stub is inserted, but if the problem
> ADRP can be replaced by ADR (i.e. the target symbol is within ±1Mb), then
> that's preferred and no stub is needed. The addition of the symbol table
> data is what then results in the stub-less form to no longer be usable in
> affected configurations. A possible countermeasure could be to move
> .rodata ahead of .text. (Also, from looking at generated code, it may well
> be that newer gcc simply avoids producing problematic insn sequences.)
>
> Linux simply compares object file sizes, but I consider that fragile: A
> change in size of one of the symbol table constituents may not necessarily
> change the object file size, due to padding which may be in use.
>
> Once we generalize linking, we may want to introduce an equivalent of
> Linux'es KALLSYMS_EXTRA_PASS as well. I don't think doing this right here
> would make overly much sense, though.
Probably we want to have the similar for RISC-V. Not so far time ago in 
downstream we faced the following issue:
https://gitlab.com/xen-project/people/olkur/xen/-/jobs/14389897308

Unfortunately I wasn't able to reproduce it locally and it was only on 
CI but I suggested the similar to this fix and it helped:
diff --git a/xen/arch/riscv/Makefile b/xen/arch/riscv/Makefile
index a1124eb5d072..72b9789cfd9c 100644
--- a/xen/arch/riscv/Makefile
+++ b/xen/arch/riscv/Makefile
@@ -52,9 +52,15 @@ $(TARGET)-syms: $(objtree)/prelink.o $(obj)/xen.lds
                | $(objtree)/tools/symbols $(all_symbols) --sysv --sort \
                > $(dot-target).2.S
        $(MAKE) $(build)=$(@D) $(dot-target).2.o
-       $(call compare-symbol-tables, $(dot-target).1.o, $(dot-target).2.o)
        $(LD) $(XEN_LDFLAGS) -T $(obj)/xen.lds $< $(build_id_linker) \
-           $(dot-target).2.o -o $@
+           $(dot-target).2.o -o $(dot-target).2
+       $(NM) -pa --format=sysv $(dot-target).2 \
+               | $(objtree)/tools/symbols $(all_symbols) --sysv --sort \
+               > $(dot-target).3.S
+       $(MAKE) $(build)=$(@D) $(dot-target).3.o
+       $(call compare-symbol-tables, $(dot-target).2.o, $(dot-target).3.o)
+       $(LD) $(XEN_LDFLAGS) -T $(obj)/xen.lds $< $(build_id_linker) \
+           $(dot-target).3.o -o $@
        $(NM) -pa --format=sysv $@ \
                | $(objtree)/tools/symbols --all-symbols --xensyms --sysv 
--sort \
                > $@.map

The fix wasn't sent as after merging some extra patches on top of it the 
issue just disappear.
> --- a/xen/arch/arm/Makefile
> +++ b/xen/arch/arm/Makefile
> @@ -99,9 +99,20 @@ $(TARGET)-syms: $(objtree)/prelink.o $(o
>                 | $(objtree)/tools/symbols $(all_symbols) --sysv --sort \
>                 > $(dot-target).2.S
>         $(MAKE) $(build)=$(@D) $(dot-target).2.o
> -       $(call compare-symbol-tables, $(dot-target).1.o, $(dot-target).2.o)
> +       if ! { $(call compare-symbol-tables, $(dot-target).1.o, $(dot-target).2.o) 
> >/dev/null; }; \
> +       then \
> +               $(LD) $(XEN_LDFLAGS) -T $(obj)/xen.lds $< $(build_id_linker) \
> +                   $(dot-target).2.o -o $(dot-target).2; \
> +               $(NM) -pa --format=sysv $(dot-target).2 \
> +                       | $(objtree)/tools/symbols $(all_symbols) --sysv --sort 
> \
> +                       > $(dot-target).3.S; \
> +               $(MAKE) $(build)=$(@D) $(dot-target).3.o; \
> +               $(call compare-symbol-tables, $(dot-target).2.o, 
> $(dot-target).3.o); \
> +       else \
> +               ln -sf $(dot-target).2.o $(dot-target).3.o; \
> +       fi
>         $(LD) $(XEN_LDFLAGS) -T $(obj)/xen.lds $< $(build_id_linker) \
> -           $(dot-target).2.o -o $@
> +           $(dot-target).3.o -o $@
>         $(NM) -pa --format=sysv $@ \
>                 | $(objtree)/tools/symbols --all-symbols --xensyms --sysv 
> --sort \
>                 > $@.map
> --- a/xen/scripts/Kbuild.include
> +++ b/xen/scripts/Kbuild.include
> @@ -65,7 +65,7 @@ define compare-symbol-tables
>       $(OBJDUMP) -t $(@D)/.cst.$$$$ > $(1).sym; \
>       ln -f $(2) $(@D)/.cst.$$$$; \
>       $(OBJDUMP) -t $(@D)/.cst.$$$$ > $(2).sym; \
> -    rm -f $(@D)/.cst.$$$$
> +    rm -f $(@D)/.cst.$$$$; \
>       diff -u $(1).sym $(2).sym
>   endef
>   
LGTM:
 Reviewed-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>

Thanks.

~ Oleksii