[PATCH] Async signal safe TLS accesses

[Rich Felker]

On Mon, Sep 23, 2013 at 04:01:29PM -0700, Andrew Hunter wrote:
> TLS accesses from initial-exec variables are async-signal-safe.  Even
> dynamic-type accesses from shared objects loaded by ld.so at startup
> are.  But dynamic accesses from dlopen()ed objects are not, which
> means a lot of trouble for any sort of per-thread state we want to
> use from signal handlers since we can't rely on always having
> initial-exec.  Make all TLS access always signal safe.
> 
> Doing this has a few components to it:
> 
>  * We introduce a set of symbols symbol_safe_{malloc,free,memalign,&c}.
>    They do what it says on the box, but guarantee async-signal-safety.
>    We provide a minimal mmap-based implementation in ld.so; anyone can
>    override them more efficiently.  (This may prove useful elsewhere.)
> [...]
>  
>      # Pointer protection.
>      __pointer_chk_guard;
> +
> +    # for signal safe TLS
> +    signal_safe_malloc; signal_safe_free; signal_safe_memalign;
> +    signal_safe_realloc; signal_safe_calloc;

These symbol names are not acceptable; they are in the space of names
reserved for the application, and since you're allowing them to be
overridden, a conforming application can cause horrible mayhem by
happening to use the same names for a different purpose. (In the worst
case, imagine an application defining signal_safe_malloc in a way that
uses TLS, such that signal_safe_malloc and __tls_get_addr become
mutually recursive...)

Rich