Re: BRSGP relocation truncations in linking kernel for Alpha.

[Michael Cree <mcree@orcon.net.nz>]

On Tue, Oct 25, 2016 at 11:07:38AM -0700, Richard Henderson wrote:
> On 10/25/2016 01:26 AM, Michael Cree wrote:
> > And while I mention gdb, it no longer works on Alpha since version
> > 7.10.  Richard, would you be able to take a look at the bug report:
> > https://sourceware.org/bugzilla/show_bug.cgi?id=19061
> 
> In the PR, Pedro has exactly the right pointer to the problem.
> 
> >From arch/alpha/kernel/traps.c:
> 
>     info.si_signo = SIGTRAP;
>     info.si_errno = 0;
>     info.si_code = TRAP_BRKPT;
>     info.si_trapno = 0;
>     info.si_addr = (void __user *) regs->pc;
> 
>     if (ptrace_cancel_bpt(current)) {
>         regs->pc -= 4;  /* make pc point to former bpt */
>     }
> 
> So we report the same si_code for executing a breakpoint insn inserted by gdb,
> and a "hardware" breakpoint managed by the kernel.  But for the later, we
> already back up the PC.
> 
> So gdb winds up backing up the PC twice.
> 
> This ought to be fixed by using TRAP_HWBKPT (4) for the ptrace_cancel_bpt case,
> but telling gdb about the issue in gdb/nat/linux-ptrace.c like so:
> 
> #elif defined __alpha__
> # define GDB_ARCH_IS_TRAP_BRKPT(X)  ((X) == TRAP_BRKPT)
> # define GDB_ARCH_IS_TRAP_HWBKPT(X) ((X) == TRAP_BRKPT || (X) == TRAP_HWBKPT)
> 
> which looks confusing, but does get checked:
> 
>     if (GDB_ARCH_IS_TRAP_BRKPT (siginfo.si_code)
>         && GDB_ARCH_IS_TRAP_HWBKPT (siginfo.si_code))
>       {
>         /* The si_code is ambiguous on this arch -- check debug
>            registers.  */
>         if (!check_stopped_by_watchpoint (lp))
>           lp->stop_reason = TARGET_STOPPED_BY_SW_BREAKPOINT;
> 
> but at the moment the default definition of GDB_ARCH_IS_TRAP_HWBKPT is always
> false for alpha.

By saying "This ought to be fixed by [...] but at the moment [...]"
are you saying that the fix provided above will not work?  Indeed,
it doesn't work: I tried it and just about every test in the gdb test
suite still fails.  For example:

Running
/home/mjc/toolchain/gdb-build/gdb/testsuite/../../../binutils-gdb/gdb/testsuite/gdb.base/break.exp
...
FAIL: gdb.base/break.exp: run until function breakpoint (timeout)
FAIL: gdb.base/break.exp: list marker1 (timeout)
FAIL: gdb.base/break.exp: break lineno (timeout)
FAIL: gdb.base/break.exp: delete $bpnum (timeout)
FAIL: gdb.base/break.exp: run until breakpoint set at a line number
(timeout)
FAIL: gdb.base/break.exp: run until file:function(6) breakpoint
(timeout)

Cheers
Michael.