Re: [PATCH] symbols: discard stray file symbols

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Apr 16, 2025 at 11:00:57AM +0200, Jan Beulich wrote:
> By observation GNU ld 2.25 may emit file symbols for .data.read_mostly
> when linking xen.efi. Due to the nature of file symbols in COFF symbol
> tables (see the code comment) the symbols_offsets[] entries for such
> symbols would cause assembler warnings regarding value truncation. Of
> course the resulting entries would also be both meaningless and useless.
> Add a heuristic to get rid of them, really taking effect only when
> --all-symbols is specified (otherwise these symbols are discarded
> anyway).
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

> ---
> Factor 2 may in principle still be too small: We zap what looks like
> real file symbols already in read_symbol(), so table_cnt doesn't really
> reflect the number of symbol table entries encountered. It has proven to
> work for me in practice though, with still some leeway left.
> 
> --- a/xen/tools/symbols.c
> +++ b/xen/tools/symbols.c
> @@ -213,6 +213,16 @@ static int symbol_valid(struct sym_entry
>       if (strstr((char *)s->sym + offset, "_compiled."))
>               return 0;
>  
> +     /* At least GNU ld 2.25 may emit bogus file symbols referencing a
> +      * section name while linking xen.efi. In COFF symbol tables the
> +      * "value" of file symbols is a link (symbol table index) to the next
> +      * file symbol. Since file (and other) symbols (can) come with one
> +      * (or in principle more) auxiliary symbol table entries, the value in
> +      * this heuristic is bounded to twice the number of symbols we have
> +      * found. See also read_symbol() as to the '?' checked for here. */
> +     if (s->sym[0] == '?' && s->sym[1] == '.' && s->addr < table_cnt * 2)

Maybe a naive question, but couldn't you drop everything below
__XEN_VIRT_START, as we shouldn't have any symbols below that
address?

Thanks, Roger.