Re: [PATCH v2] Arm: do a 4th linking pass if necessary

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

Hi Jan,

> On 21 May 2026, at 14:02, Jan Beulich <jbeulich@xxxxxxxx> 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>

Acked-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>

Cheers
Bertrand

> ---
> 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.
> ---
> v2: Add "set -e".
> 
> --- a/xen/arch/arm/Makefile
> +++ b/xen/arch/arm/Makefile
> @@ -99,9 +99,21 @@ $(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 \
> + set -e; \
> + $(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
>