Re: [PATCH 2/2] gdb/testsuite: Accept PIE/noPIE programs in gdb.mi/mi-var-invalidate-shlib.exp

[Tom de Vries via Gdb-patches <gdb-patches@sourceware.org>]

On 8/2/22 14:47, Lancelot SIX wrote:
> PR/29426 shows failures when running the gdb.mi/mi-var-invalidate-shlib
> test when using a compiler which does not produce a PIE executable by
> default.
> 
> In the testcase, a varobj is created to track a global variable, and
> then the main binary is reloaded in GDB (using the file command).
> 
> During the load of the new binary, GDB tries to recreate the varobj to
> track the global in the new binary (varobj_invalidate_iter).  At this
> point, the old process is still in flight.  So when we try to access to
> the value of the global, in a PIE executable we only have access to the
> unrelocated address (the objfile's text_section_offset () is 0).  As a
> consequence down the line read_value_memory fails to read the unrelated
> address, so cannot evaluate the value of the global.  Note that the
> expression used to access to the global’s value is valid, so the varobj
> can be created.  When using a non PIE executable, the address of the
> global GDB knows about at this point does not need relocation, so
> read_value_memory can access the (old binary’s) value.
> 
> So at this point, in the case of a non-PIE executable the value field is
> set, while it is cleared in the case of PIE executable.  Later when the
> test issues a "-var-update global_var", the command sees no change in
> the case of the non-PIE executable, while in the case of the PIE
> executable install_new_value sees that value changes, leading to a
> different output.
> 
> This patch changes the testcase to accept both behaviors.

Hi,

thanks for the analysis.

After looking into this a bit, I wonder if the root cause is really 
looking at the values in the old process.

Maybe we should either:
- not try to sample the values when recreating, or
- move the recreation to a later point when we have access to the memory
   of the new process.

At least we would have consistency between PIE and no-PIE with either 
solution.

Thanks,
- Tom