Re: glibc strerrorname_np

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 08/11/2021 19:22, Jonny Grant wrote:
> 
> 
> On 06/11/2021 12:51, Adhemerval Zanella wrote:
>>
>>
>> On 05/11/2021 19:23, Jonny Grant wrote:
>>>
>>>
>>> On 05/11/2021 13:01, Adhemerval Zanella wrote:
>>>>
>>>>
>>>> On 05/11/2021 08:51, Jonny Grant wrote:
>>>>>>
>>>>>
>>>>> Hi Adhemerval
>>>>>
>>>>> Thank you for your reply. Personally I understood an ABI break would be the return type, the name, or the parameters. But the proposed change is not so. Changing to return a string, should be fine.
>>>>
>>>> It is still an ABI break, code that checks NULL for invalid input will
>>>> stop to work.
>>>>
>>>>>
>>>>> ie, in relation to strerror() C99 and POSIX.1-2008 require the return value to be non-NULL. (my view is it is always better not to return a NULL from such string functions that could then cause a SEGV.
>>>>>
>>>>> strerror(1000) returns a string "Unknown error 1000"
>>>>>
>>>>> Better to simply align with glibc strerror() approach?
>>>>>
>>>>> Feels like there is still time to change it, as it is _np. Aligning with strerror(), or just "" as you had mentioned seems reasonable.
>>>>
>>>> I give you that it is indeed a better return code, and it is not a matter
>>>> of timing, but rather I don't think it really worth the ABI break and 
>>>> the required code complexity to do so. 
>>>>
>>>> It would require:
>>>>
>>>>   1. Change the strerrorname_np to return "" on invalid code.
>>>
>>> Please find attached the patch.
>>>
>>>>   2. Keep the compat symbol that returns NULL and add a compat symbol.
>>>>   3. Exports a new symbol with version on 2.35 with the new semantic
>>>>      and update the ailist.
> 
> Could you dircet me to the ailist file you mention please.

I spelled it wrong, it should be 'abilist':

$ find . -iname *.abilist
./sysdeps/unix/sysv/linux/x86_64/64/libBrokenLocale.abilist
./sysdeps/unix/sysv/linux/x86_64/64/libm.abilist
./sysdeps/unix/sysv/linux/x86_64/64/libutil.abilist
[...]

You can check if the exported symbols are on par with the pre-defined
ones with:

$ make check-abi

And you can update the abilist files with:

$ make update-abi

It will need to be properly exported on the 'Versions' file.


> 
>>>
>>> May I check, why would a new symbol be needed? I'd expect it is only a change to strerrorname_np and any test code you have that presently checks for NULL return.
>>
>> As I said before it is an ABI break, since users that check for invalid 
>> errno against NULL will start to fail.  For such change we *do need* all 
>> the trouble of adding a compat symbol with current semantic.
> Many thanks for your reply. May I check, are even the _np functions set in stone once they are released? glibc strerrorname_np was released a year ago. My disto Ubuntu LTS doesn't yet have this new glibc release containing this function.

It is not a matter of time, neither glibc is tied to any distribution.  It is
a matter or point release, for instance 2.34.  Once we export such symbol on
a release, programs will be built against and depend of the symbol.

> 
>>>
>>>>   4. Update the documentation and sync with man-pages. 
>>>
>>> The man-page update is minor, I could handle that.
>>>
>>>
>>>>> https://man7.org/linux/man-pages/man3/strerror.3.html
>>>>>
>>>>> It's common for some returns to change, eg glibc 2.13 changed strerror_r() behaviour to return the actual error code, as opposed to returning -1 and setting errno.
>>>>
>>>> And such change did got without burden and extra complexity. Just check
>>>> the multiple preprocessor checks it requires to get the right definition
>>>> depending of the system support on the misc/error.c (imported from gnulib).
>>>>
>>>
>>> Ok, I think the change I propose does not affect the definition, as the function signature is the same. Maybe I misunderstand something.
>>
>> The ABI break is not only for function signature and input alignment/size,
>> but also for function semantic.  Just check the fmemopen
>> (fdb7d390dd0d96e4a8239c46f3aa64598b90842b), where we kept the old buggy 
>> implementation since even when it is not POSIX compliant because we do
>> not know if users do depend of such behavior.
>>
> 
> I was reading the fmemopen ticket you shared. May I ask - could you direct me to a "compat symbol" description? I did search and came across libc_hidden_proto and see it in use in /glibc/include/stdio.h the patch commit note says update fmemopen to the new POSIX spec.

To put simply, the 'compat symbol' is a GNU extension that creates a 
symbol tied to an specific version where a linked program built against 
an older glibc will continue to use the older version instead of the 
default one.

There is some information on how it is created internally on
'include/shlib-compat.h'.