* Re: Random gdbserver question (x86)
2010-03-31 17:35 ` Michael Snyder
@ 2010-03-31 17:43 ` Pedro Alves
2010-03-31 17:46 ` Michael Snyder
2010-03-31 17:48 ` Daniel Jacobowitz
2010-03-31 17:57 ` Mark Kettenis
2 siblings, 1 reply; 9+ messages in thread
From: Pedro Alves @ 2010-03-31 17:43 UTC (permalink / raw)
To: Michael Snyder; +Cc: gdb
On Wednesday 31 March 2010 18:35:29, Michael Snyder wrote:
> And is that more-or-less true of gdb as well? I.e., if host/target
> is x86_64-linux, then gdb can debug both 32 and 64 bit apps, but if
> host/target is i386-linux, then gdb can only debug 32 bit apps?
Native gdb yes. A 32-bit GDB should still be able to debug a
remote/gdbserver 64-bit inferior.
>
> Actually, I phrase it as a question even though I am pretty sure it
> is true. My real question is "Why?". It seems to me that if we just
> included amd64-linux-nat.o and amd64-*-tdep.o in the build, then
> 32-bit gdb should be able to debug 64-bit native apps.
Wouldn't work. The limitation is on ptrace.
--
Pedro Alves
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Random gdbserver question (x86)
2010-03-31 17:43 ` Pedro Alves
@ 2010-03-31 17:46 ` Michael Snyder
0 siblings, 0 replies; 9+ messages in thread
From: Michael Snyder @ 2010-03-31 17:46 UTC (permalink / raw)
To: Pedro Alves; +Cc: gdb
Pedro Alves wrote:
> On Wednesday 31 March 2010 18:35:29, Michael Snyder wrote:
>> And is that more-or-less true of gdb as well? I.e., if host/target
>> is x86_64-linux, then gdb can debug both 32 and 64 bit apps, but if
>> host/target is i386-linux, then gdb can only debug 32 bit apps?
>
> Native gdb yes. A 32-bit GDB should still be able to debug a
> remote/gdbserver 64-bit inferior.
>
>> Actually, I phrase it as a question even though I am pretty sure it
>> is true. My real question is "Why?". It seems to me that if we just
>> included amd64-linux-nat.o and amd64-*-tdep.o in the build, then
>> 32-bit gdb should be able to debug 64-bit native apps.
>
> Wouldn't work. The limitation is on ptrace.
Can it be explained relatively simply? ;-)
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Random gdbserver question (x86)
2010-03-31 17:35 ` Michael Snyder
2010-03-31 17:43 ` Pedro Alves
@ 2010-03-31 17:48 ` Daniel Jacobowitz
2010-03-31 17:56 ` Michael Snyder
2010-03-31 17:57 ` Mark Kettenis
2 siblings, 1 reply; 9+ messages in thread
From: Daniel Jacobowitz @ 2010-03-31 17:48 UTC (permalink / raw)
To: Michael Snyder; +Cc: Pedro Alves, gdb
On Wed, Mar 31, 2010 at 10:35:29AM -0700, Michael Snyder wrote:
> Actually, I phrase it as a question even though I am pretty sure it
> is true. My real question is "Why?". It seems to me that if we just
> included amd64-linux-nat.o and amd64-*-tdep.o in the build, then
> 32-bit gdb should be able to debug 64-bit native apps.
I don't think the necessary kernel interfaces exist. Or at least, we
aren't using them. Compare the offsets in the two nat files.
--
Daniel Jacobowitz
CodeSourcery
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Random gdbserver question (x86)
2010-03-31 17:48 ` Daniel Jacobowitz
@ 2010-03-31 17:56 ` Michael Snyder
0 siblings, 0 replies; 9+ messages in thread
From: Michael Snyder @ 2010-03-31 17:56 UTC (permalink / raw)
To: Michael Snyder, Pedro Alves, gdb
Daniel Jacobowitz wrote:
> On Wed, Mar 31, 2010 at 10:35:29AM -0700, Michael Snyder wrote:
>> Actually, I phrase it as a question even though I am pretty sure it
>> is true. My real question is "Why?". It seems to me that if we just
>> included amd64-linux-nat.o and amd64-*-tdep.o in the build, then
>> 32-bit gdb should be able to debug 64-bit native apps.
>
> I don't think the necessary kernel interfaces exist. Or at least, we
> aren't using them. Compare the offsets in the two nat files.
>
Hmmm, but both nat files get linked in when we do a 64 bit build.
Suppose I wanted to build gdb on a 32 bit host, but use it on a
64 bit host. If I specify "target=x86_64-linux", then my gdb will
not work natively on *either* host, because it will be built as a
cross-only debugger.
^ permalink raw reply [flat|nested] 9+ messages in thread
* Re: Random gdbserver question (x86)
2010-03-31 17:35 ` Michael Snyder
2010-03-31 17:43 ` Pedro Alves
2010-03-31 17:48 ` Daniel Jacobowitz
@ 2010-03-31 17:57 ` Mark Kettenis
2 siblings, 0 replies; 9+ messages in thread
From: Mark Kettenis @ 2010-03-31 17:57 UTC (permalink / raw)
To: msnyder; +Cc: pedro, gdb
> Date: Wed, 31 Mar 2010 10:35:29 -0700
> From: Michael Snyder <msnyder@vmware.com>
>
> Pedro Alves wrote:
> > On Wednesday 31 March 2010 00:40:34, Michael Snyder wrote:
> >> Question -- is gdbserver "multi-arch" to the extent that a 32-bit
> >> gdbserver (built with target = i386) can debug a 64-bit app?
> >
> > No, only the other way around. It's "multi-arch" to the extent
> > that a linux 64-bit gdbserver (built with target = x86-64) can
> > debug a 32-bit app. It used to only be, only 64x64 or 32x32 was
> > possible; with multi-arch, 64x32 was made possible.
> >
> >> I ask because linux-x86-low.c is full of ifdefs on the architecture.
> >> It looks like most of the 64-bit code won't be compiled if the
> >> build machine is 32-bit.
> >
> > Correct.
>
> And is that more-or-less true of gdb as well? I.e., if host/target
> is x86_64-linux, then gdb can debug both 32 and 64 bit apps, but if
> host/target is i386-linux, then gdb can only debug 32 bit apps?
>
> Actually, I phrase it as a question even though I am pretty sure it
> is true. My real question is "Why?". It seems to me that if we just
> included amd64-linux-nat.o and amd64-*-tdep.o in the build, then
> 32-bit gdb should be able to debug 64-bit native apps.
The problem here is that ptrace(2) from a 32-bit executable only gives
you access to the 32-bit registers. Debugging 64-bit executables
remotely should work fine though, provided you --enable-64-bit-bfd.
^ permalink raw reply [flat|nested] 9+ messages in thread