On Sat, Feb 15, 2025 at 6:21 AM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote: > > 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. This seems like a useful change all by itself even while the main feature of this patch set is still being developed and reviewed. Should we land just this .kallsyms_rodata change? > > > > -- > Best Regards > Masahiro Yamada
