Re: [PATCH v3 06/16] asprintf_chk: Ensure compatibility for both s390x and ppc64le

[Paul E Murphy <murphyp@linux.ibm.com>]

On 6/28/23 3:42 AM, Frédéric Bérat via Libc-alpha wrote:
> If ldbl_* macros are used for asprintf, ABI gets broken on s390x,
> if it isn't, ppc64le isn't building due to multiple asm redirections.
> 
> This is due to the inclusion of bits/stdio-lbdl.h for ppc64le whereas it
> isn't for s390x. This header creates redirections, which are not
> compatible with the ones generated using libc_hidden_builtin_def.
> Yet, we can't use libc_hidden_ldbl_proto on s390x since it will not
> create a simple strong alias (e.g. as done on x86_64), but a versioned
> alias, leading to ABI breakage.
> ---
>   debug/asprintf_chk.c | 12 ++++++++++++
>   include/stdio.h      |  5 +++++
>   2 files changed, 17 insertions(+)
> 
> diff --git a/debug/asprintf_chk.c b/debug/asprintf_chk.c
> index 06d112de00..7da1cddacb 100644
> --- a/debug/asprintf_chk.c
> +++ b/debug/asprintf_chk.c
> @@ -36,5 +36,17 @@ ___asprintf_chk (char **result_ptr, int flag, const char *format, ...)
> 
>     return ret;
>   }
> +#if defined __LDBL_COMPAT || __LDOUBLE_REDIRECTS_TO_FLOAT128_ABI == 1 > +/* This is needed since <bits/stdio-lbdl.h> is included in this 
case, leading to
> + * multiple asm redirection of the same symbol
> + */
>   ldbl_hidden_def (___asprintf_chk, __asprintf_chk)
>   ldbl_strong_alias (___asprintf_chk, __asprintf_chk)
> +#else
> +/* Yet, we can't use ldbl_* macros on some systems, even if they don't fall in
> + * the first case as this leads to ABI breakage due to the long_double_symbol
> + * aliasing, which is versionned.

s/versionned/versioned/

It has been a few years since I really looked at this macros, why only 
asprintf and not the other printf functions?  I find the ABI comment 
hard to follow, could it include more details about how it would be 
broken by using the ldbl_* macros?

Also, does patchset 5 in this series break ppc64le or s390x build 
without this patch?  This seems like it should be squashed with the 
previous patch.

> + */
> +strong_alias (___asprintf_chk, __asprintf_chk)
> +libc_hidden_builtin_def (__asprintf_chk)
> +#endif
> diff --git a/include/stdio.h b/include/stdio.h
> index 3afb0ff77b..6755877911 100644
> --- a/include/stdio.h
> +++ b/include/stdio.h
> @@ -279,7 +279,12 @@ rtld_hidden_proto (__libc_fatal)
> 
>   libc_hidden_proto (__fgets_unlocked_chk)
> 
> +#if defined __LDBL_COMPAT || __LDOUBLE_REDIRECTS_TO_FLOAT128_ABI == 1
>   libc_hidden_ldbl_proto (__asprintf_chk)
> +#else
> +libc_hidden_proto (__asprintf_chk)
> +#endif
> +
>   libc_hidden_ldbl_proto (__fprintf_chk)
>   libc_hidden_ldbl_proto (__sprintf_chk)
>   libc_hidden_ldbl_proto (__vsprintf_chk)