Re: [PATCH v1 1/3] xen/riscv: fix switch_stack_and_jump()

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 5/19/26 11:28 AM, Jan Beulich wrote:
> On 19.05.2026 10:39, Oleksii Kurochko wrote:
> > The following compilation issue occurs when UBSAN related stuff is enabled:
> > prelink.o: in function `smp_processor_id':
> >    /build/xen/./arch/riscv/include/asm/current.h:46:(.init.text+0x274e2):
> >    relocation truncated to fit: R_RISCV_JAL against `init_done'
> > make[2]: *** [arch/riscv/Makefile:45: xen-syms] Error 1
> There's no init_done() as of yet.
It was found based on downstream version of RISC-V port.

> > The switch_stack_and_jump macro uses "j " #fn which assembles to
> > JAL x0, init_done is a RISC-V J-type instruction with only ±1MB range.
> >
> > Without UBSAN, .init.text is small enough that init_done (which lives in
> > .text, not .init.text) is within 1MB of the JAL. With UBSAN enabled, all
> > the instrumentation calls bloat .init.text well past 1MB, so init_done
> > is now >1MB away from the JAL. The linker tries to truncate the 20-bit
> > J-type offset and fails.
> .init.text is well below 64k right now. Are you telling us that it grows
> by more than a factor of 16 when UBSAN is enabled? IOW while the change
> may indeed be needed, I question this explanation. .text growth may matter
> as well, and e.g. .rodata (living between both sections) might also grow.
No, it won't grow so much.

With UBSAN enabled:

$ objdump -h xen/prelink.o

xen/prelink.o:     file format elf64-little

Sections:
Idx Name          Size      VMA               LMA               File off
  0 .text         0011c79e  0000000000000000  0000000000000000  00000040
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
  1 .init.text    000285fe  0000000000000000  0000000000000000  0011c7de
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE

With UBSAN, .text itself is 0x11c79e ≈ 1.11 MiB — already exceeding the 
JAL range on its own. Even if .init.text directly followed .text (which 
it doesn't), a call from .init.text to a symbol near the start of .text 
would be ~1.11 MiB away. init_done likely sits somewhere specific within 
.text rather than at its very end, but add the .rodata + .data sections 
on top and the gap is comfortably past ±1 MiB.
As a result, the target symbol init_done may end up outside the range 
supported by the R_RISCV_JAL relocation, which is limited to 
approximately ±1 MiB.
Without UBSAN enabled:

xen/prelink.o:     file format elf64-little

Sections:
Idx Name          Size      VMA               LMA               File off
  0 .text         00044618  0000000000000000  0000000000000000  00000040
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
  1 .init.text    00012c72  0000000000000000  0000000000000000  00044658
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE

Does it make sense now? I can use the text above for commit message 
instead of what is mentioned now for more accuracy.
Would it be better to send this patch when this issue will occur in 
upstream?
~ Oleksii