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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 13/03/2025 3:39 pm, Jan Beulich wrote:
> 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.

It's GCC-11 sadly.  cf_check is part of the function type, and triggers
when a function type check would be relevant.  Just casing to an integer
won't trigger it, I don't think.

~Andrew