On Fri, Feb 7, 2025 at 10:21 AM Stephen Brennan <stephen.s.brennan@xxxxxxxxxx> wrote: > > When vmlinux is linked, the rodata from kallsyms is placed arbitrarily > within the .rodata section. The linking process is repeated several > times, since the kallsyms data size changes, which shifts symbols, > requiring re-generating the data and re-linking. > > BTF is generated during the first link only. For variables, BTF includes > a BTF_K_DATASEC for each data section that may contain a variable, which > includes the variable's name, type, and offset within the data section. > Because the size of kallsyms data changes during later links, the > offsets of variables placed after it in .rodata will change. This means > that BTF_K_DATASEC information for those variables becomes inaccurate. > > This is not currently a problem, because BTF currently only generates > variable data for percpu variables. However, the next commit will add > support for generating BTF for all global variables, including for the > .rodata section. > > We could re-generate BTF each time vmlinux is linked, but this is quite > expensive, and should be avoided at all costs. Further as each chunk of > data (BTF and kallsyms) are re-generated, there's no guarantee that > their sizes will converge anyway. > > Instead, we can take advantage of the fact that BTF only cares to store > the offset of variables from the start of their section. Therefore, so > long as the kallsyms data is stored last in the .rodata section, no > offsets will be affected. Adjust kallsyms to output to .rodata.kallsyms, > and update the linker script to include this at the end of .rodata. > > Signed-off-by: Stephen Brennan <stephen.s.brennan@xxxxxxxxxx> > --- I am fine if this is helpful for BTF. -- Best Regards Masahiro Yamada
