Re: [PATCH v2] gdb/arm: Remove tpidruro register from non-FreeBSD target descriptions

[John Baldwin <jhb@FreeBSD.org>]

On 2/27/24 7:10 PM, Thiago Jung Bauermann wrote:
> Commit 92d48a1e4eac ("Add an arm-tls feature which includes the tpidruro
> register from CP15.") introduced the org.gnu.gdb.arm.tls feature, which
> adds the tpidruro register, and unconditionally enabled it in
> aarch32_create_target_description.
> 
> In Linux, the tpidruro register isn't available via ptrace in the 32-bit
> kernel but it is available for an aarch32 program running under an arm64
> kernel via the ptrace compat interface.  This isn't currently implemented
> however, which causes GDB on arm-linux with 64-bit kernel to list the
> register but show it as unavailable, as reported by Tom de Vries:
> 
>    $ gdb -q -batch a.out -ex start -ex 'p $tpidruro'
>    Temporary breakpoint 1 at 0x512
> 
>    Temporary breakpoint 1, 0xaaaaa512 in main ()
>    $1 = <unavailable>
> 
> Simon Marchi then clarified:
> 
>> The only time we should be seeing some "unavailable" registers or memory
>> is in the context of tracepoints, for things that are not collected.
>> Seeing an unavailable register here is a sign that something is not
>> right.
> 
> Therefore, disable the TLS feature in aarch32 target descriptions for Linux
> and NetBSD targets (the latter also doesn't seem to support accessing
> tpidruro either, based on a quick look at arm-netbsd-nat.c).
> 
> This patch fixes the following tests:
> 
> Running gdb.base/inline-frame-cycle-unwind.exp ...
> FAIL: gdb.base/inline-frame-cycle-unwind.exp: cycle at level 3: backtrace when the unwind is broken at frame 3
> FAIL: gdb.base/inline-frame-cycle-unwind.exp: cycle at level 5: backtrace when the unwind is broken at frame 5
> FAIL: gdb.base/inline-frame-cycle-unwind.exp: cycle at level 1: backtrace when the unwind is broken at frame 1
> 
> Tested with Ubuntu 22.04.3 on armv8l-linux-gnueabihf in native,
> native-gdbserver and native-extended-gdbserver targets with no regressions.
> 
> PR tdep/31418
> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31418
> ---
> 
> Hello,
> 
> As often happens, after I sent v1 of this patch I noticed a problem with it:
> It introduces two variants of aarch32 target description (one with TLS
> support and one without), but only creates and returns the first one that was
> requested.  This version fixes the problem.
> 
> The only change compared to v1 is in gdb/aarch32-tdep.c.
> 
> Changes in v2:
> - Cache two versions of tdesc_aarch32 in aarch32_read_description, one with
> TLS support and one without.t

Good catch, V2 looks ok to me as well.

Approved-By: John Baldwin <jhb@FreeBSD.org>

-- 
John Baldwin