Re: [PATCH v2 3/5] symbols: arrange to know where functions end

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.08.2025 09:28, Jan Beulich wrote:
> On 28.08.2025 03:03, Jason Andryuk wrote:
>> On 2025-04-02 09:58, Jan Beulich wrote:
>>> --- a/xen/tools/symbols.c
>>> +++ b/xen/tools/symbols.c
>>
>>> @@ -318,24 +334,42 @@ static void write_src(void)
>>>     printf("#else\n");
>>>     output_label("symbols_offsets");
>>>     printf("#endif\n");
>>> -   for (i = 0; i < table_cnt; i++) {
>>> +   for (i = 0, ends = 0; i < table_cnt; i++) {
>>>             printf("\tPTR\t%#llx - SYMBOLS_ORIGIN\n", table[i].addr);
>>> +
>>> +           table[i].addr_idx = i + ends;
>>> +
>>> +           if (!want_symbol_end(i)) {
>>> +                   /* If there's another symbol at the same address,
>>> +                    * propagate this symbol's size if the next one has
>>> +                    * no size, or if the next one's size is larger. */
>>
>> Why do we want to shrink the next symbol's size?
> 
> First (see related post-commit-message remarks): In principle section symbols
> could come with a size, too. That would break everything as long as we don't
> strip those.
> 
> The main reason though is that imo smallest granularity is what we want here,
> together with predictability. One symbol with a huge size could cover
> multiple other symbols with smaller sizes. We could omit that part of the
> change here, but then the processing in the hypervisor would need to change,
> to fish out the "best suitable" symbol when dealing with multiple ones at the
> same address. Other changes may then also be needed to the tool, to have such
> symbols come in a well-defined order (to keep the then-new code in the
> hypervisor as simple as possible). Look for "aliased symbol" in
> common/symbols.c to see how simplistic respective code is right now.

Furthermore remember that we can't record sizes, but instead we insert fake
symbols. Obviously there can be only one (at least in the present scheme).
If we used too large a size, chances would increase that the end symbol (in
the sorted table) would have to live past some other symbol, thus becoming
that one's "end".

Jan