[PATCH] elf: Support at least 32-byte alignment in static dlopen

[Carlos O'Donell]

On 7/10/20 4:26 PM, Florian Weimer via Libc-alpha wrote:
> Otherwise loading a dynamically linked libc with rseq support fails,
> as result of the __rseq_abi TLS variable, which has an alignment
> of 32 bytes.

OK for 2.32.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

> ---
>  csu/libc-tls.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/csu/libc-tls.c b/csu/libc-tls.c
> index 06e76bd395..3f1655f264 100644
> --- a/csu/libc-tls.c
> +++ b/csu/libc-tls.c
> @@ -112,6 +112,12 @@ __libc_setup_tls (void)
>    size_t tcb_offset;
>    const ElfW(Phdr) *phdr;
>  
> +  /* libc.so with rseq has TLS with 32-byte alignment.  Static dlopen
> +     requires at least 32-byte alignment as well, otherwise loading
> +     libc.so will always fail.  */
> +  if (max_align < 32)
> +    max_align = 32;

OK. Same problem as the previous discuss, namely that you'd need to look
over all the modules you're going to dlopen first to compute a value
that is large enough. All we're doing here is papering over a design problem,
but that is OK for 2.32. Eventually we're going to need to do some kind of
redesign here to make this more reliable.

> +
>    struct link_map *main_map = GL(dl_ns)[LM_ID_BASE]._ns_loaded;
>  
>    /* Look through the TLS segment if there is any.  */
> 


-- 
Cheers,
Carlos.