Re: [PATCH v3 3/4] scripts/faddr2line: Constrain readelf output to symbols from System.map

[Sami Tolvanen]

On Tue, Aug 1, 2023 at 9:42 AM Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
>
> On Fri, Jul 28, 2023 at 4:34 AM Will Deacon <will@xxxxxxxxxx> wrote:
> >
> > Some symbols emitted in the readelf output but filtered from System.map
> > can confuse the 'faddr2line' symbol size calculation, resulting in the
> > erroneous rejection of valid offsets. This is especially prevalent when
> > building an arm64 kernel with CONFIG_CFI_CLANG=y, where most functions
> > are prefixed with a 32-bit data value in a '$d.n' section. For example:
> >
> > 447538: ffff800080014b80   548 FUNC    GLOBAL DEFAULT    2 do_one_initcall
> >    104: ffff800080014c74     0 NOTYPE  LOCAL  DEFAULT    2 $x.73
> >    106: ffff800080014d30     0 NOTYPE  LOCAL  DEFAULT    2 $x.75
> >    111: ffff800080014da4     0 NOTYPE  LOCAL  DEFAULT    2 $d.78
> >    112: ffff800080014da8     0 NOTYPE  LOCAL  DEFAULT    2 $x.79
> >     36: ffff800080014de0   200 FUNC    LOCAL  DEFAULT    2 run_init_process
>
> Sami,
> Should we change the llvm-ir linkage type for these symbols from
> `internal` to `private`?
> https://llvm.org/docs/LangRef.html#linkage-types
>
> Then they would not appear in the symbol table.
>
> At first, I thought other modules might need to directly reference
> this data, but with the local binding, I don't think they can.

For arm64, we don't explicitly emit symbols for type prefixes (see
AsmPrinter::emitKCFITypeId). These mapping symbols are emitted by
AArch64ELFStreamer to mark data and code regions. According to the
AArch64 ELF specification, "All mapping symbols have type STT_NOTYPE
and binding STB_LOCAL," so I assume changing the linkage type isn't an
option:

https://github.com/ARM-software/abi-aa/blob/main/aaelf64/aaelf64.rst#mapping-symbols

Sami