Re: [PATCH v3 1/2] [gdb/symtab] Fix an out of bounds array access in find_epilogue_using_linetable

[Bernd Edlinger <bernd.edlinger@hotmail.de>]

On 4/5/24 17:10, Tom de Vries wrote:
> From: Bernd Edlinger <bernd.edlinger@hotmail.de>
> 
> An out of bounds array access in find_epilogue_using_linetable causes random
> test failures like these:
> 
> FAIL: gdb.base/unwind-on-each-insn-amd64.exp: foo: instruction 6: $fba_value == $fn_fba
> FAIL: gdb.base/unwind-on-each-insn-amd64.exp: foo: instruction 6: check frame-id matches
> FAIL: gdb.base/unwind-on-each-insn-amd64.exp: foo: instruction 6: bt 2
> FAIL: gdb.base/unwind-on-each-insn-amd64.exp: foo: instruction 6: up
> FAIL: gdb.base/unwind-on-each-insn-amd64.exp: foo: instruction 6: $sp_value == $::main_sp
> FAIL: gdb.base/unwind-on-each-insn-amd64.exp: foo: instruction 6: $fba_value == $::main_fba
> FAIL: gdb.base/unwind-on-each-insn-amd64.exp: foo: instruction 6: [string equal $fid $::main_fid]
> 
> Here the read happens below the first element of the line
> table, and the test failure depends on the value that is
> read from there.
> 
> It also happens that std::lower_bound returns a pointer exactly at the upper
> bound of the line table, also here the read value is undefined, that happens
> in this test:
> 
> FAIL: gdb.dwarf2/dw2-epilogue-begin.exp: confirm watchpoint doesn't trigger
> 
> Fixes: 528b729be1a2 ("gdb/dwarf2: Add support for DW_LNS_set_epilogue_begin in line-table")
> 
> Co-Authored-By: Tom de Vries <tdevries@suse.de>
> 
> PR symtab/31268
> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31268
> ---
>  gdb/symtab.c | 85 +++++++++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 75 insertions(+), 10 deletions(-)
> 
> diff --git a/gdb/symtab.c b/gdb/symtab.c
> index 86603dfebc3..0c126d99cd4 100644
> --- a/gdb/symtab.c
> +++ b/gdb/symtab.c
> @@ -4166,10 +4166,14 @@ find_epilogue_using_linetable (CORE_ADDR func_addr)
>  	= unrelocated_addr (end_pc - objfile->text_section_offset ());
>  
>        const linetable *linetable = sal.symtab->linetable ();
> -      /* This should find the last linetable entry of the current function.
> -	 It is probably where the epilogue begins, but since the DWARF 5
> -	 spec doesn't guarantee it, we iterate backwards through the function
> -	 until we either find it or are sure that it doesn't exist.  */
> +      if (linetable->nitems == 0)
> +	{
> +	  /* Empty line table.  */
> +	  return {};
> +	}
> +
> +      /* Find the first linetable entry after the current function.  Note that
> +	 this also may be an end_sequence entry.  */
>        auto it = std::lower_bound
>  	(linetable->item, linetable->item + linetable->nitems, unrel_end,
>  	 [] (const linetable_entry &lte, unrelocated_addr pc)
> @@ -4177,13 +4181,74 @@ find_epilogue_using_linetable (CORE_ADDR func_addr)
>  	   return lte.unrelocated_pc () < pc;
>  	 });
>  
> -      while (it->unrelocated_pc () >= unrel_start)
> -      {
> -	if (it->epilogue_begin)
> -	  return {it->pc (objfile)};
> -	it --;
> -      }
> +      if (it == linetable->item + linetable->nitems)
> +	{
> +	  /* We couldn't find either:
> +	     - a linetable entry starting the function after the current
> +	       function, or
> +	     - an end_sequence entry that terminates the current function
> +	       at unrel_end.
> +	     This can happen when the linetable doesn't describe the full
> +	     extent of the function.  Even though this is a corner case, which
> +	     may not happen other than in dwarf assembly test-cases, let's
> +	     handle this.
> +
> +	     Move to the last entry in the linetable, and check that it's an
> +	     end_sequence terminating the current function.  */
> +	  gdb_assert (it != &linetable->item[0]);
> +	  it--;
> +	  if (!(it->line == 0
> +		&& unrel_start <= it->unrelocated_pc ()
> +		&& it->unrelocated_pc () < unrel_end))
> +	    return {};

Why is this check necessary here, and not also when
this is not the last function in the line-table?

And why is this check necessary at all?

> +	}
> +      else
> +	gdb_assert (unrel_end <= it->unrelocated_pc ());

Why do you not check that 'it' points to an end_sequence
at exactly unrel_end?
It could be anything at an address much higher PC than unrel_end.


Bernd.