Re: [PATCH 1/6] symbols: add minimal self-test

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.03.2025 16:35, Andrew Cooper wrote:
> On 13/03/2025 1:52 pm, Jan Beulich wrote:
>> ... before making changes to the involved logic.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> With this FAST_SYMBOL_LOOKUP may make sense to permit enabling even
>> when LIVEPATCH=n. Thoughts? (In this case "symbols: centralize and re-
>> arrange $(all_symbols) calculation" would want pulling ahead.)
>>
>> --- a/xen/common/symbols.c
>> +++ b/xen/common/symbols.c
>> @@ -260,6 +260,41 @@ unsigned long symbols_lookup_by_name(con
>>      return 0;
>>  }
>>  
>> +#ifdef CONFIG_SELF_TESTS
>> +
>> +static void __init test_lookup(unsigned long addr, const char *expected)
>> +{
>> +    char buf[KSYM_NAME_LEN + 1];
>> +    const char *name, *symname;
>> +    unsigned long size, offs;
>> +
>> +    name = symbols_lookup(addr, &size, &offs, buf);
>> +    if ( !name )
>> +        panic("%s: address not found\n", expected);
>> +    if ( offs )
>> +        panic("%s: non-zero offset (%#lx) unexpected\n", expected, offs);
>> +
>> +    /* Cope with static symbols, where varying file names/paths may be 
>> used. */
>> +    symname = strchr(name, '#');
>> +    symname = symname ? symname + 1 : name;
>> +    if ( strcmp(symname, expected) )
>> +        panic("%s: unexpected symbol name: '%s'\n", expected, symname);
>> +
>> +    offs = symbols_lookup_by_name(name);
>> +    if ( offs != addr )
>> +        panic("%s: address %#lx unexpected; wanted %#lx\n",
>> +              expected, offs, addr);
>> +}
>> +
>> +static void __init __constructor test_symbols(void)
>> +{
>> +    /* Be sure to only try this for cf_check functions. */
> 
> I'm very happy to see the take-up of SELF_TESTs.  Although I probably
> ought to tie it into a Kconfig option to make the errors non-fatal,
> which I've been meaning to do for a bit.
> 
> One question though.  cf_check is an x86-ism, even if it leaks out into
> common code.
> 
> I think you mean "functions emitted into the final image"?  If so, I
> don't think this is relevant then, because ...
> 
>> +    test_lookup((unsigned long)dump_execstate, "dump_execstate");
>> +    test_lookup((unsigned long)test_symbols, __func__);
> 
> ... taking the function address here forces it to be emitted even if it
> would otherwise have been inlined.

No, I really mean cf_check. If we took the address of a non-cf_check
function, the special gcc13 build's checking would trigger, aiui.

Jan