Re: [RFC] Make "run" work on macOS 10.13

[Simon Marchi <simon.marchi@polymtl.ca>]

On 2018-06-29 16:55, Tom Tromey wrote:
> I would like some feedback on this patch.
> 
> On macOS 10.13.5, "run" does not work in gdb.  There are two cases:
> 
> 1. If I forget to "set startup-with-shell off", then gdb will fail due
>    to the system integrity protection feature.  I believe this happens
>    because gdb is not allowed to debug the shell.
> 
>    You can find many sites advocating "set startup-with-shell off",
>    but it seems to me that it is friendlier for gdb to simply do it by
>    default.
> 
>    One option here might be to do this conditionally based on the
>    version of the OS.
> 
> 2. I found that gdb was setting the solib breakpoint incorrectly,
>    causing a failure.  Adding the load address to the notifier address
>    makes this work for me.  I suspect this would regress earlier
>    versions of macOS, but I have no way to test that; one idea might
>    be to only do this when gdb_dyld_all_image_infos::version == 15.
> 
> gdb/ChangeLog
> 2018-06-29  Tom Tromey  <tom@tromey.com>
> 
> 	* solib-darwin.c (darwin_solib_create_inferior_hook): Create solib
> 	breakpoint later.  Add load_addr to the notifier address.
> 	* darwin-nat.c (darwin_nat_target::create_inferior): Bind
> 	startup_with_shell to 0.
> ---
>  gdb/ChangeLog      | 7 +++++++
>  gdb/darwin-nat.c   | 5 +++++
>  gdb/solib-darwin.c | 9 +++++----
>  3 files changed, 17 insertions(+), 4 deletions(-)
> 
> diff --git a/gdb/ChangeLog b/gdb/ChangeLog
> index 4c04d0ba728..c6462259fe0 100644
> --- a/gdb/ChangeLog
> +++ b/gdb/ChangeLog
> @@ -1,3 +1,10 @@
> +2018-06-29  Tom Tromey  <tom@tromey.com>
> +
> +	* solib-darwin.c (darwin_solib_create_inferior_hook): Create solib
> +	breakpoint later.  Add load_addr to the notifier address.
> +	* darwin-nat.c (darwin_nat_target::create_inferior): Bind
> +	startup_with_shell to 0.
> +
>  2018-06-28  Tom Tromey  <tom@tromey.com>
> 
>  	* NEWS: Mention --enable-codesign.
> diff --git a/gdb/darwin-nat.c b/gdb/darwin-nat.c
> index 7dccce73926..542c8389ef0 100644
> --- a/gdb/darwin-nat.c
> +++ b/gdb/darwin-nat.c
> @@ -1809,6 +1809,11 @@ darwin_nat_target::create_inferior (const char
> *exec_file,
>  				    const std::string &allargs,
>  				    char **env, int from_tty)
>  {
> +  /* Starting with Sierra, SIP prevents gdb from attaching to the
> +     shell, so users have to disable startup-with-shell.  */
> +  scoped_restore save_startup
> +    = make_scoped_restore (&startup_with_shell, 0);
> +
>    /* Do the hard work.  */
>    fork_inferior (exec_file, allargs, env, darwin_ptrace_me,

I think this part is good.  I would suggest printing a message/warnings 
to indicate that we are disabling startup-with-shell (only if 
startup_with_shell is 1 in the first place).

>  		 darwin_ptrace_him, darwin_pre_ptrace, NULL,
> diff --git a/gdb/solib-darwin.c b/gdb/solib-darwin.c
> index ed8e0c13365..a4e15dc6b5b 100644
> --- a/gdb/solib-darwin.c
> +++ b/gdb/solib-darwin.c
> @@ -528,10 +528,6 @@ darwin_solib_create_inferior_hook (int from_tty)
>        return;
>      }
> 
> -  /* Add the breakpoint which is hit by dyld when the list of solib is
> -     modified.  */
> -  create_solib_event_breakpoint (target_gdbarch (), 
> info->all_image.notifier);
> -
>    if (info->all_image.count != 0)
>      {
>        /* Possible relocate the main executable (PIE).  */
> @@ -547,6 +543,11 @@ darwin_solib_create_inferior_hook (int from_tty)
>        load_addr = darwin_read_exec_load_addr_at_init (info);
>      }
> 
> +  /* Add the breakpoint which is hit by dyld when the list of solib is
> +     modified.  */
> +  create_solib_event_breakpoint (target_gdbarch (),
> +				 info->all_image.notifier + load_addr);
> +
>    if (load_addr != 0 && symfile_objfile != NULL)
>      {
>        CORE_ADDR vmaddr;

About the dynamic loader relocation, I am trying to compare your 
approach with Xavier's approach here:

https://sourceware.org/ml/gdb-patches/2018-08/msg00519.html

If I print the resulting notifier address, I get two different values:

With Tom's patch: 0x10000f782
With Xavier's patch: 0x100012782

The unrelocated value of the symbol is 0xf782.  That breakpoint is used 
for "set stop-on-solib-events", it seems, so I tried to enable that with 
both of your patches.  I got a stop with Xavier's patch and none with 
Tom's, which leads me to think that Xavier's patch gets it right.  I 
think you may be using the executable base address, while we actually 
want to use dyld's base address?  This is not very clear to me yet.

Simon