Re: [PATCH] Fix #27777 - now use a doubly-linked list for _IO_list_all

[alexandre.ferrieux@orange.com]

On 26/04/2024 19:51, Florian Weimer wrote:
> 
> * alexandre ferrieux:
> 
>> diff --git a/libio/bits/types/struct_FILE.h b/libio/bits/types/struct_FILE.h
>> index 7cdaae86f8..daebd6ce0a 100644
>> --- a/libio/bits/types/struct_FILE.h
>> +++ b/libio/bits/types/struct_FILE.h
>> @@ -67,7 +67,7 @@ struct _IO_FILE
>>  
>>    struct _IO_marker *_markers;
>>  
>> -  struct _IO_FILE *_chain;
>> +  struct _IO_FILE *_chain,**_prevchain;
>>  
>>    int _fileno;
>>    int _flags2;
> 
> Unfortunately, this struct is part of the ABI for several reasons.  For
> example, _IO_2_1_stdin_ is a global variable exported by libc.so.6, and
> it has a struct _IO_FILE member.  It cannot change size due to copy
> relocations, even if nothing accesses members after _chain.
> 
> I disagree with H.J.'s suggestion that the answer is symbol
> versions. 8-)

Interesting !

So we are in the presence of software that can no longer change, _ever_ ?
What about bumping to _IO_2_2 ?

> Two options come to my mind: Use the FILE * value itself as a hash table
> key (and use _chain for resolving bucket conflicts), or use a dynarray
> and replace _chain with a union of a pointer and an unsigned int,
> containing the dynarray index for that particular file.  The latter has
> the advantage that we can use it to implement arbitrary extensions in
> the future.  Either approach should preserve compatibility with GCC 2.95
> libstdc++ if we are a bit careful.

Argggh. Jumping through complex, inefficient and hard-to-maintain hoops,
because a fundamentally opaque type has been inadvertently exposed ?

Imagine the state of _IO_FILE 50 years from now, after a dozen of similar hacks 
have piled up...

Are there absolutely no plans to someday deprecate the problematic, but 
extremely rare, accesses to those types that should have remained opaque ?



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