gdb: ensure bp_location::section is set correct to avoid an assert

While reviewing and testing another patch I set a breakpoint on an
gnu ifunc function, then restarted the inferior, and this assert
triggered:

  ../../src/gdb/breakpoint.c:14747: internal-error: breakpoint_free_objfile: Assertion `loc->symtab == nullptr' failed.

The backtrace at the time of the assert is:

  bminor#6  0x00000000005ffee0 in breakpoint_free_objfile (objfile=0x4064b30) at ../../src/gdb/breakpoint.c:14747
  bminor#7  0x0000000000c33ff2 in objfile::~objfile (this=0x4064b30, __in_chrg=<optimized out>) at ../../src/gdb/objfiles.c:478
  bminor#8  0x0000000000c38da6 in std::default_delete<objfile>::operator() (this=0x7ffc1a49d538, __ptr=0x4064b30) at /usr/include/c++/9/bits/unique_ptr.h:81
  bminor#9  0x0000000000c3782a in std::unique_ptr<objfile, std::default_delete<objfile> >::~unique_ptr (this=0x7ffc1a49d538, __in_chrg=<optimized out>) at /usr/include/c++/9/bits/unique_ptr.h:292
  bminor#10 0x0000000000caf1bd in owning_intrusive_list<objfile, intrusive_base_node<objfile> >::erase (this=0x3790d68, i=...) at ../../src/gdb/../gdbsupport/owning_intrusive_list.h:111
  bminor#11 0x0000000000cacd0c in program_space::remove_objfile (this=0x3790c80, objfile=0x4064b30) at ../../src/gdb/progspace.c:192
  bminor#12 0x0000000000c33e1c in objfile::unlink (this=0x4064b30) at ../../src/gdb/objfiles.c:408
  bminor#13 0x0000000000c34fb9 in objfile_purge_solibs (pspace=0x3790c80) at ../../src/gdb/objfiles.c:729
  bminor#14 0x0000000000edf6f7 in no_shared_libraries (pspace=0x3790c80) at ../../src/gdb/solib.c:1359
  bminor#15 0x0000000000fb3f6c in target_pre_inferior () at ../../src/gdb/target.c:2466
  bminor#16 0x0000000000a724d7 in run_command_1 (args=0x0, from_tty=0, run_how=RUN_NORMAL) at ../../src/gdb/infcmd.c:390
  bminor#17 0x0000000000a72a97 in run_command (args=0x0, from_tty=0) at ../../src/gdb/infcmd.c:514
  #18 0x00000000006bbb3d in do_simple_func (args=0x0, from_tty=0, c=0x39124b0) at ../../src/gdb/cli/cli-decode.c:95
  #19 0x00000000006c1021 in cmd_func (cmd=0x39124b0, args=0x0, from_tty=0) at ../../src/gdb/cli/cli-decode.c:2827

The function breakpoint_free_objfile is being called when an objfile
representing a shared library is being unloaded ahead of the inferior
being restarted, the function is trying to remove references to
anything that could itself reference the objfile that is being
deleted.

The assert is making the claim that, for a bp_location, which has a
single address, the objfile of the symtab associated with the location
will be the same as the objfile associated with the section of the
location.

This seems reasonable to me now, as it did when I added the assert in
commit:

  commit 5066f36
  Date:   Mon Nov 11 21:45:17 2024 +0000

      gdb: do better in breakpoint_free_objfile

The bp_location::section is maintained, according to the comments in
breakpoint.h, to aid overlay debugging (is that even used any more),
and looking at the code, this does appear to be the case.

The problem in the above case arises when we are dealing with an ifunc
function.  What happens is that we end up with a section from one
objfile, but a symtab from a different objfile.

This problem originates from minsym_found (in linespec.c).  The user
asked for 'break gnu_ifunc' where 'gnu_ifunc' is an ifunc function.
What this means is that gnu_ifunc is actually a resolver function that
returns the address of the actual function to use.

In this particular test case, the resolver function is in a shared
library, and the actual function to use is in the main executable.

So, when GDB looks for 'gnu_ifunc' is finds the minimal_symbol with
that name, and spots that this has type mst_text_gnu_ifunc.  GDB then
uses this to figure out the actual address of the function that will
be run.

GDB then creates the symtab_and_line using the _real_ address and the
symtab in which that address lies, in our case this will all be
related to the main executable objfile.

But, finally, in minsym_found, GDB fills in the symtab_and_line's
section field, and this is done using the section containing the
original minimal_symbol, which is from the shared library objfile.

The minimal symbol and section are then use to initialise the
bp_location object, and this is how we end up in, what I think, is an
unexpected state.

So what to do about this?

The symtab_and_line::msymbol field is _only_ set within minsym_found,
and is then _only_ used to initialise the bp_location::msymbol field.

The bp_location::msymbol field is _only_ used in the function
set_breakpoint_location_function, and we only really care about the
msymbol type, we check to see if it's an ifunc symbol or not.  This
allows us to set the name of the function correctly.

The bp_location::section is used, as far as I can tell, extensively
for overlay handling.  It would seem to me, that this section should
be the section containing the actual breakpoint address.  If the
question we're asking is, is this breakpoint mapped in or not?  Then
surely we need to ask about the section holding the breakpoint's
address, and not the section holding some other code (e.g. the
resolver function).  In fact, in a memory constrained environment,
you'd expect the resolver functions to get mapped out pretty early on,
but while the actual functions might still be mapped in.

Finally, symtab_and_line::section.  This is mostly set using calls to
find_pc_overlay.  The minsym_found function is one of the few places
where we do things differently.  In the places where the section is
used, it is (almost?) always used in conjunction with the
symtab_and_line::pc to lookup information, e.g. calls to
block_for_pc_sect, or find_pc_sect_containing_function.  In all these
cases, it appears to me that the assumption is that the section will
be the section that contains the address.

So, where does this leave us?

I think what we need to do is update minsym_found to just use
find_pc_overlay, which is how the symtab_and_line::section is set in
most other cases.  What this actually means in practise is that the
section field will be set to NULL (see find_pc_overlay in symfile.c).
But given that this is how the section is computed in most other
cases, I don't see why it should be especially problematic for this
case.  In reality, I think this just means that the section is
calculated via a call to find_pc_section when it's needed, as an
example, see lookup_minimal_symbol_by_pc_section (minsyms.c).

I do wonder if we should be doing better when creating the
symtab_and_line, and insist that the section be calculated correctly
at that point, but I really don't want to open that can of worms right
now, so I think just changing minsym_found to "do it just like
everyone else" should be good enough.

I've extended the existing ifunc test to expose this issue, the
updated test fails without this patch, and passes with.

Approved-By: Simon Marchi <simon.marchi@efficios.com>

Author: T-J-Teru

gdb/linespec.c

@@ -4113,7 +4113,12 @@ minsym_found (struct linespec_state *self, struct objfile *objfile,
       sal.pspace = current_program_space;
     }
 
-  sal.section = msymbol->obj_section (objfile);
+  /* Don't use the section from the msymbol, the code above might have
+     adjusted FUNC_ADDR, in which case the msymbol's section might not be
+     the section containing FUNC_ADDR.  It might not even be in the same
+     objfile.  As the section is primarily to assist with overlay
+     debugging, it should reflect the SAL's pc value.  */
+  sal.section = find_pc_overlay (sal.pc);
 
   if (self->maybe_add_address (objfile->pspace (), sal.pc))
     add_sal_to_sals (self, result, &sal, msymbol->natural_name (), false);

gdb/testsuite/gdb.base/gnu-ifunc.exp

@@ -185,6 +185,11 @@ proc_with_prefix set-break {resolver_attr resolver_debug final_debug} {
 	# other two locations.
 	gdb_test "info breakpoints" "$location\r\n.*$location\r\n$location"
     }
+
+    # At one point a bug existed such that GDB would trigger an assert
+    # while restarting the inferior with ifunc breakpoints set.
+    gdb_run_cmd
+    gdb_test "" "Breakpoint $::decimal,.*final \\(\[^\r\n\]*\\).*" "restart, run until breakpoint"
 }
 
 # Misc GNU ifunc tests.  For the description of RESOLVER_ATTR,