Re: [PATCH] symbols: discard stray file symbols

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.10.2025 11:56, Roger Pau Monné wrote:
> 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>

Thanks.

>> --- 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?

If we assumed so, then that might be an option. Such an assumption doesn't
look safe to me, though. See how e.g. hv_hcall_page is outside of the Xen
image (albeit still within __XEN_VIRT_{START,END}). I wouldn't want to
preclude architectures playing "interesting" games with symbols, in
particular when - unlike x86 - they have the entire VA space for their use.

Jan