mtrace: Deprecate mallwatch and tr_break

The variable and function pair appear to provide a way for users to
set conditional breakpoints in mtrace when a specific address is
returned by the allocator.  This can be achieved by using conditional
breakpoints in gdb so it is redundant.  There is no documentation of
this interface in the manual either, so it appears to have been a hack
that got added to debug malloc.  Deprecate these symbols and do not
call tr_break anymore.
---
 NEWS            |  4 ++++
 malloc/mtrace.c | 57 +++++++++++++++++--------------------------------
 2 files changed, 24 insertions(+), 37 deletions(-)

On 6/23/21 9:25 PM, Siddhesh Poyarekar via Libc-alpha wrote:
> The variable and function pair appear to provide a way for users to
> set conditional breakpoints in mtrace when a specific address is
> returned by the allocator.  This can be achieved by using conditional
> breakpoints in gdb so it is redundant.  There is no documentation of
> this interface in the manual either, so it appears to have been a hack
> that got added to debug malloc.  Deprecate these symbols and do not
> call tr_break anymore.
> ---
>   NEWS            |  4 ++++
>   malloc/mtrace.c | 57 +++++++++++++++++--------------------------------
>   2 files changed, 24 insertions(+), 37 deletions(-)
> 
> diff --git a/NEWS b/NEWS
> index cadc40262f..536e80721a 100644
> --- a/NEWS
> +++ b/NEWS
> @@ -64,6 +64,10 @@ Deprecated and removed features, and other changes affecting compatibility:
>   * The function pthread_yield has been deprecated; programs should use
>     the equivalent standard function sched_yield instead.
>   
> +* The symbols mallwatch and tr_break are now deprecated and no longer used in
> +  mtrace.  Similar functionality can be achieved by using conditional
> +  breakpoints within mtrace functions from within gdb.
> +
>   Changes to build and runtime requirements:
>   
>   * On Linux, the shm_open, sem_open, and related functions now expect the
> diff --git a/malloc/mtrace.c b/malloc/mtrace.c
> index b65b21a933..6c2c58b706 100644
> --- a/malloc/mtrace.c
> +++ b/malloc/mtrace.c
> @@ -50,8 +50,25 @@ static char *malloc_trace_buffer;
>   
>   __libc_lock_define_initialized (static, lock);
>   
> -/* Address to breakpoint on accesses to... */
> +#if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_34)
> +/* Compatibility symbols that were introduced to help break at allocation sites
> +   for specific memory allocations.  This is unusable with ASLR, although gdb
> +   may allow predictable allocation addresses.  Even then, gdb has watchpoint
> +   and conditional breakpoint support which should provide the same
> +   functionality without having this kludge.  These symbols are preserved in
> +   case some applications ended up linking against them but they don't actually
> +   do anything anymore; not that they did much before anyway.  */
> +
>   void *mallwatch;
> +compat_symbol (libc, mallwatch, mallwatch, GLIBC_2_0);
> +
> +void
> +tr_break (void)
> +{
> +}
> +compat_symbol (libc, tr_break, tr_break, GLIBC_2_0);
> +#endif
> +
>   
>   /* Old hook values.  */
>   static void (*tr_old_free_hook) (void *ptr, const void *);
> @@ -61,19 +78,6 @@ static void *(*tr_old_realloc_hook) (void *ptr, size_t size,
>   static void *(*tr_old_memalign_hook) (size_t __alignment, size_t __size,
>   				      const void *);
>   
> -/* This function is called when the block being alloc'd, realloc'd, or
> -   freed has an address matching the variable "mallwatch".  In a debugger,
> -   set "mallwatch" to the address of interest, then put a breakpoint on
> -   tr_break.  */
> -
> -extern void tr_break (void) __THROW;
> -libc_hidden_proto (tr_break)
> -void
> -tr_break (void)
> -{
> -}
> -libc_hidden_def (tr_break)
> -
>   static void
>   tr_where (const void *caller, Dl_info *info)
>   {
> @@ -167,12 +171,6 @@ tr_freehook (void *ptr, const void *caller)
>     tr_where (caller, info);
>     /* Be sure to print it first.  */
>     fprintf (mallstream, "- %p\n", ptr);
> -  if (ptr == mallwatch)
> -    {
> -      __libc_lock_unlock (lock);
> -      tr_break ();
> -      __libc_lock_lock (lock);
> -    }
>     set_default_hooks ();
>     if (tr_old_free_hook != NULL)
>       (*tr_old_free_hook)(ptr, caller);
> @@ -203,9 +201,6 @@ tr_mallochook (size_t size, const void *caller)
>   
>     __libc_lock_unlock (lock);
>   
> -  if (hdr == mallwatch)
> -    tr_break ();
> -
>     return hdr;
>   }
>   
> @@ -214,9 +209,6 @@ tr_reallochook (void *ptr, size_t size, const void *caller)
>   {
>     void *hdr;
>   
> -  if (ptr == mallwatch)
> -    tr_break ();
> -
>     Dl_info mem;
>     Dl_info *info = lock_and_info (caller, &mem);
>   
> @@ -247,9 +239,6 @@ tr_reallochook (void *ptr, size_t size, const void *caller)
>   
>     __libc_lock_unlock (lock);
>   
> -  if (hdr == mallwatch)
> -    tr_break ();
> -
>     return hdr;
>   }
>   
> @@ -274,9 +263,6 @@ tr_memalignhook (size_t alignment, size_t size, const void *caller)
>   
>     __libc_lock_unlock (lock);
>   
> -  if (hdr == mallwatch)
> -    tr_break ();
> -
>     return hdr;
>   }
>   
> @@ -296,10 +282,7 @@ release_libc_mem (void)
>   #endif
>   
>   
> -/* We enable tracing if either the environment variable MALLOC_TRACE
> -   is set, or if the variable mallwatch has been patched to an address
> -   that the debugging user wants us to stop on.  When patching mallwatch,
> -   don't forget to set a breakpoint on tr_break!  */
> +/* We enable tracing if the environment variable MALLOC_TRACE is set.  */
>   
>   void
>   mtrace (void)
> @@ -321,7 +304,7 @@ mtrace (void)
>   #else
>     mallfile = getenv (mallenv);
>   #endif
> -  if (mallfile != NULL || mallwatch != NULL)
> +  if (mallfile != NULL)
>       {
>         char *mtb = malloc (TRACE_BUFFER_SIZE);
>         if (mtb == NULL)
>