Re: Debugging on Linux with non-fragile ABI?

[David Chisnall]

Hi,

Neither debugger currently knows anything about the non-fragile ABI. 
The Apple runtimes have a few introspection functions that are called 
without acquiring the necessary locks, which are intended to be called 
by the debugger, but someone needs to write the relevant code in either 
gdb or lldb and let me know what hooks they'd like.  The lldb code is 
relatively approachable and the Objective-C runtime interfaces are quite 
well abstracted, so it would be fairly simple to add.

Note that lldb probably does not give you the correct layout, especially 
for anything that is not a base class.  It looks as if it's relying on 
the compiler's guess of where ivars will be, which is not always correct 
in the non-fragile ABI.

Materialising ivar offsets is relatively easy.  The change is basically 
to use a different symbol name for the ivar offsets.

If no one has the time or inclination to do this, then I'd recommend 
calling the runtime introspection functions directly from the {g,ll}db 
command line.  These will give you the correct layouts.  It's a little 
bit clunky (though would be easy to wrap in a plug-in function), but 
these can give you the type and offset of each ivar and let you write 
some pointer arithmetic that shows you the correct ivar.

David

On 18/11/2019 09:12, Frederik Seiffert wrote:
> Hi Wolfgang,
>
> We’re running into very similar issues debugging using lldb with the GNUstep 
> Android toolchain, which is using the modern (v2) ABI. Have you been able to 
> get any further with this?
>
> The following lldb command seems to indicate that the root cause might be lldb 
> missing knowledge about the ObjC runtime. Could you check if you get the same 
> output for this?
>
> > (lldb) language objc class-table dump
> > error: current process has no Objective-C runtime loaded
>
> Thanks,
> Frederik
>
>
> > Am 18.06.2019 um 02:49 schrieb Wolfgang Lux <address@hidden>:
> >
> > Hi all,
> >
> > has anybody tips for how to debug GNUstep executables built with the
> > non-fragile ABI?
> > Gdb seems pretty useless with the non-fragile ABI because all instance variable
> > offsets appear to be 0 so it's impossible to inspect the contents of any
> > instances:
> > (gdb) print *self
> > $1 = {<> = {isa = 0x6074f8 <_OBJC_CLASS_Test>},
> >    name = 0x6074f8 <_OBJC_CLASS_Test>, value = 0x6074f8 <_OBJC_CLASS_Test>,
> >    parent = 0x6074f8 <_OBJC_CLASS_Test>, children = 0x6074f8 <_OBJC_CLASS_Test>}
> >
> > I've also tried lldb, which at least understands the object layout:
> > (lldb) print *self
> > (Test) $0 = {
> >    NSObject = {
> >      isa = 0x00000000006074f8
> >    }
> >    name = 0x0000000000607588
> >    value = 0x0000000000607588
> >    parent = nil
> >    children = 0x00000000007949b8
> > }
> > However, when I try to inspect any of the instance attributes I only get an
> > error message:
> > Error [IRForTarget]: Couldn't find Objective-C indirect ivar symbol
> > OBJC_IVAR_$_Test.name
> > error: The expression could not be prepared to run in the target
> > (lldb) print self->value
> > Error [IRForTarget]: Couldn't find Objective-C indirect ivar symbol
> > OBJC_IVAR_$_Test.value
> > error: The expression could not be prepared to run in the target
> >
> > I've checked this on Ubuntu 18.04 with lldb 6.0 (and before any FreeBSD
> > advocates run in, it's exactly the same on FreeBSD 11.3 which comes with lldb
> > 8.0)
> >
> > All of this works fine on macOS:
> > (lldb) print *self
> > (Test) $0 = {
> >    NSObject = {
> >      isa = Test
> >    }
> >    name = 0x0000000100003118 @"$"
> >    value = 0x0000000100003118 @"$"
> >    parent = nil
> >    children = 0x0000000100305910 2 key/value pairs
> > }
> > (lldb) print *self->name
> > (NSString) $1 = {
> >    NSObject = {
> >      isa = __NSCFConstantString
> >    }
> > }
> > (lldb) print self->name
> > (__NSCFConstantString *) $2 = 0x0000000100003118 @"$"
> >
> > Wolfgang