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

[alexandre.ferrieux@orange.com]

On 26/04/2024 20:50, H.J. Lu wrote:
>
> On Fri, Apr 26, 2024 at 10:51 AM Florian Weimer <fweimer@redhat.com> 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-)
> 
> libio/bits/types/struct_FILE.h has
> 
>    __off64_t _offset;
>    /* Wide character stream stuff.  */
>    struct _IO_codecvt *_codecvt;
>    struct _IO_wide_data *_wide_data;
>    struct _IO_FILE *_freeres_list;
>    void *_freeres_buf;
>    size_t __pad5;
>    int _mode;
>    /* Make sure we don't get into trouble again.  */
>    char _unused2[15 * sizeof (int) - 4 * sizeof (void *) - sizeof (size_t)];
> };
> 
> We can add a pointer without changing FILE size.

Ha ! That's a clever trick. Presumably someone was already bitten once ;)
(Sorry I didn't notice it was present in your corrected patch).

Now, I still have worries about the symbol versioning approach: why restrict the 
version bump to _IO_un_link and _IO_link_in ? Indeed, any program calling 
fclose()  ends up calling _IO_un_link(), so the version check should apply too, 
right ? So shouldn't we tag all functions using (FILE *) in their arguments or 
return value ?
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