[PATCH] RISC-V: Allow setting breakpoints at invalid addresses.

[Andrew Burgess]

* Jim Wilson <jimw@sifive.com> [2019-04-13 13:42:42 -0700]:

> Some testsuite testcases construct dwarf2 debug info for fake functions to
> test that this debug info is handled correctly.  We get an error trying to
> read from an invalid address when setting a breakpoint on these fake functions.
> Other targets allow setting breakpoints on invalid addresses, and only error
> when a command forces us to write the breakpoint to memory.  This matches that
> behavior by wrapping the read in a try/catch.
> 
> Tested with a riscv64-linux native testsuite run.  I get 55 fewer unexpected
> failures with the patch, and there are no regressions.
> 
> 	gdb/
> 	* riscv-tdep.c (riscv_breakpoint_kind_from_pc): Wrap read_code call
> 	in try/catch.  Set buf[0] to 0 in catch clause.
> ---
>  gdb/riscv-tdep.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/gdb/riscv-tdep.c b/gdb/riscv-tdep.c
> index 6370bc268f..38dc578add 100644
> --- a/gdb/riscv-tdep.c
> +++ b/gdb/riscv-tdep.c
> @@ -430,8 +430,19 @@ riscv_breakpoint_kind_from_pc (struct gdbarch *gdbarch, CORE_ADDR *pcptr)
>  	unaligned_p = true;
>        else
>  	{
> -	  /* Read the opcode byte to determine the instruction length.  */
> -	  read_code (*pcptr, buf, 1);
> +	  try
> +	    {
> +	      /* Read the opcode byte to determine the instruction length.  */
> +	      read_code (*pcptr, buf, 1);
> +	    }
> +	  catch (const gdb_exception_error &ex)
> +	    {
> +	      /* We may have tried to set a breakpoint at a invalid address.
> +		 Defer the error until we try to write the breakpoint to
> +		 memory to match how other gdb targets work.  Also, testsuite
> +		 testcases like gdb.cp/nsalias.exp require this behavior.  */
> +	      buf[0] = 0;
> +	    }

I think that you can just use something like:

	  /* Read the opcode byte to determine the instruction length.  If
	     the read fails this may be because we tried to set the
	     breakpoint at an invalid address.  We provide a fake result
	     which will give a breakpoint length of 4, hopefully when we
	     try to actually insert the breakpoint we will see a failure
	     then too.  */
	  if (target_read_code (*pcptr, buf, 1) == -1)
	    buf[0] = 0;

and avoid the try/catch.

Annoyingly my QEMU/RISC-V/Linux setup got broken so I'm unable to test
the above right now ... hopefully I'll get some time next week to fix
my infrastructure issue :-/

Thanks,
Andrew

>  	}
>  
>        if (riscv_debug_breakpoints)
> -- 
> 2.17.1
>