Really fail inserting software breakpoints on read-only regions

Really fail inserting software breakpoints on read-only regions

Currently, with "set breakpoint auto-hw off", we'll still try to
insert a software breakpoint at addresses covered by supposedly
read-only or inacessible regions:

 (top-gdb) mem 0x443000 0x450000 ro
 (top-gdb) set mem inaccessible-by-default off
 (top-gdb) disassemble
 Dump of assembler code for function main:
    0x0000000000443956 <+34>:    movq   $0x0,0x10(%rax)
 => 0x000000000044395e <+42>:    movq   $0x0,0x18(%rax)
    0x0000000000443966 <+50>:    mov    -0x24(%rbp),%eax
    0x0000000000443969 <+53>:    mov    %eax,-0x20(%rbp)
 End of assembler dump.
 (top-gdb) b *0x0000000000443969
 Breakpoint 5 at 0x443969: file ../../src/gdb/gdb.c, line 29.
 (top-gdb) c
 Continuing.
 warning: cannot set software breakpoint at readonly address 0x443969

 Breakpoint 5, 0x0000000000443969 in main (argc=1, argv=0x7fffffffd918) at ../../src/gdb/gdb.c:29
 29        args.argc = argc;
 (top-gdb)

We warn, saying that the insertion can't be done, but then proceed
attempting the insertion anyway, and in case of manually added
regions, the insert actually succeeds.

This is a regression; GDB used to fail inserting the breakpoint.  More
below.

I stumbled on this as I wrote a test that manually sets up a read-only
memory region with the "mem" command, in order to test GDB's behavior
with breakpoints set on read-only regions, even when the real memory
the breakpoints are set at isn't really read-only.  I wanted that in
order to add a test that exercises software single-stepping through
read-only regions.

Note that the memory regions that target_memory_map returns aren't
like e.g., what would expect to see in /proc/PID/maps on Linux.
Instead, they're the physical memory map from the _debuggers_
perspective.  E.g., a read-only region would be real ROM or flash
memory, while a read-only+execute mapping in /proc/PID/maps is still
read-write to the debugger (otherwise the debugger wouldn't be able to
set software breakpoints in the code segment).

If one tries to manually write to memory that falls within a memory
region that is known to be read-only, with e.g., "p foo = 1", then we
hit a check in memory_xfer_partial_1 before the write mananges to make
it to the target side.

But writing a software/memory breakpoint nowadays goes through
target_write_raw_memory, and unlike when writing memory with
TARGET_OBJECT_MEMORY, nothing on the TARGET_OBJECT_RAW_MEMORY path
checks whether we're trying to write to a read-only region.

At the time "breakpoint auto-hw" was added, we didn't have the
TARGET_OBJECT_MEMORY vs TARGET_OBJECT_RAW_MEMORY target object
distinction yet, and the code path in memory_xfer_partial that blocks
writes to read-only memory was hit for memory breakpoints too.  With
GDB 6.8 we had:

 warning: cannot set software breakpoint at readonly address 0000000000443943
 Warning:
 Cannot insert breakpoint 1.
 Error accessing memory address 0x443943: Input/output error.

So I started out by fixing this by adding the memory region validation
to TARGET_OBJECT_RAW_MEMORY too.

But later, when testing against GDBserver, I realized that that would
only block software/memory breakpoints GDB itself inserts with
gdb/mem-break.c.  If a target has a to_insert_breakpoint method, the
insertion request will still pass through to the target.  So I ended
up converting the "cannot set breakpoint" warning in breakpoint.c to a
real error return, thus blocking the insertion sooner.

With that, we'll end up no longer needing the TARGET_OBJECT_RAW_MEMORY
changes once software single-step breakpoints are converted to real
breakpoints.  We need them today as software single-step breakpoints
bypass insert_bp_location.  But, it'll be best to leave that in as
safeguard anyway, for other direct uses of TARGET_OBJECT_RAW_MEMORY.

Tested on x86_64 Fedora 20, native and gdbserver.

gdb/
2014-10-01  Pedro Alves  <palves@redhat.com>

	* breakpoint.c (insert_bp_location): Error out if inserting a
	software breakpoint at a read-only address.
	* target.c (memory_xfer_check_region): New function, factored out
	from ...
	(memory_xfer_partial_1): ... this.  Make the 'reg_len' local a
	ULONGEST.
	(target_xfer_partial) <TARGET_OBJECT_RAW_MEMORY>: Check the access
	against the memory region attributes.

gdb/testsuite/
2014-10-01  Pedro Alves  <palves@redhat.com>

	* gdb.base/breakpoint-in-ro-region.c: New file.
	* gdb.base/breakpoint-in-ro-region.exp: New file.