Re: [PATCH] symbols: discard stray file symbols

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.09.2025 23:53, Jason Andryuk wrote:
> On 2025-04-16 05:00, 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>
>> ---
>> 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)
>> +            return 0;
>> +
>>      return 1;
>>   }
> 
> I looked at this.  It'll drop symbols, but I don't know enough to give 
> an R-b.  I can't give an actionable A-b either.   Maybe someone else can 
> chime in.
> 
> Maybe this is just showing my lack of knowledge, but could any symbol 
> starting "?." be considered invalid?  I don't think I've ever seen any 
> like that.

With quotation, almost any symbol name can appear in principle. I wouldn't
want to judge symbol validity by its name. What's more important here,
though, is that sym[0] isn't part of the name; it's the symbol's type as
taken from nm's output. We're therefore heuristically looking at symbols
of unknown type with a dot as the first character (as section names would
conventionally have it).

Jan