[v2,1/5] Prefer symtab symbol over minsym for function names in non-contiguous blocks

Message ID 20190704045503.1250-2-kevinb@redhat.com
State New
Headers show
Series
  • Non-contiguous address range bug fixes / improvements
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Commit Message

Kevin Buettner July 4, 2019, 4:54 a.m.
The discussion on gdb-patches which led to this patch may be found
here:

https://www.sourceware.org/ml/gdb-patches/2019-05/msg00018.html

Here's a brief synopsis/analysis:

Eli Zaretskii, while debugging a Windows emacs executable, found
that functions comprised of more than one (non-contiguous)
address range were not being displayed correctly in a backtrace.  This
is the example that Eli provided:

  (gdb) bt
  #0  0x76a63227 in KERNELBASE!DebugBreak ()
     from C:\Windows\syswow64\KernelBase.dll
  #1  0x012e7b89 in emacs_abort () at w32fns.c:10768
  #2  0x012e1f3b in print_vectorlike.cold () at print.c:1824
  #3  0x011d2dec in print_object (obj=<optimized out>, printcharfun=XIL(0),
      escapeflag=true) at print.c:2150

The function print_vectorlike consists of two address ranges, one of
which contains "cold" code which is expected to not execute very often.
There is a minimal symbol, print_vectorlike.cold.65, which is the address
of the "cold" range.

GDB is prefering this minsym over the the name provided by the
DWARF info due to some really old code in GDB which handles
"certain pathological cases".  This comment reads as follows:

      /* In certain pathological cases, the symtabs give the wrong
	 function (when we are in the first function in a file which
	 is compiled without debugging symbols, the previous function
	 is compiled with debugging symbols, and the "foo.o" symbol
	 that is supposed to tell us where the file with debugging
	 symbols ends has been truncated by ar because it is longer
	 than 15 characters).  This also occurs if the user uses asm()
	 to create a function but not stabs for it (in a file compiled
	 with -g).

	 So look in the minimal symbol tables as well, and if it comes
	 up with a larger address for the function use that instead.
	 I don't think this can ever cause any problems; there
	 shouldn't be any minimal symbols in the middle of a function;
	 if this is ever changed many parts of GDB will need to be
	 changed (and we'll create a find_pc_minimal_function or some
	 such).  */

In an earlier version of this patch, I had left the code for the
pathological case intact, but those who reviwed that patch recommended
removing it.  So that's what I've done - I've removed it.

gdb/ChangeLog:

	* stack.c (find_frame_funname): Remove code which preferred
	minsym over symtab sym in "certain pathological cases".
---
 gdb/stack.c | 71 +++++++++++------------------------------------------
 1 file changed, 15 insertions(+), 56 deletions(-)

-- 
2.21.0

Comments

Kevin Buettner July 4, 2019, 5:03 a.m. | #1
I find the diff easier to read when the -w switch is used.  So here's
just the patch using -w:

diff --git a/gdb/stack.c b/gdb/stack.c
index b3d113d3b4..e0a7403ec0 100644
--- a/gdb/stack.c
+++ b/gdb/stack.c
@@ -1134,46 +1134,6 @@ find_frame_funname (struct frame_info *frame, enum language *funlang,
 
   func = get_frame_function (frame);
   if (func)
-    {
-      /* In certain pathological cases, the symtabs give the wrong
-         function (when we are in the first function in a file which
-         is compiled without debugging symbols, the previous function
-         is compiled with debugging symbols, and the "foo.o" symbol
-         that is supposed to tell us where the file with debugging
-         symbols ends has been truncated by ar because it is longer
-         than 15 characters).  This also occurs if the user uses asm()
-         to create a function but not stabs for it (in a file compiled
-         with -g).
-
-         So look in the minimal symbol tables as well, and if it comes
-         up with a larger address for the function use that instead.
-         I don't think this can ever cause any problems; there
-         shouldn't be any minimal symbols in the middle of a function;
-         if this is ever changed many parts of GDB will need to be
-         changed (and we'll create a find_pc_minimal_function or some
-         such).  */
-
-      struct bound_minimal_symbol msymbol;
-
-      /* Don't attempt to do this for inlined functions, which do not
-	 have a corresponding minimal symbol.  */
-      if (!block_inlined_p (SYMBOL_BLOCK_VALUE (func)))
-	msymbol
-	  = lookup_minimal_symbol_by_pc (get_frame_address_in_block (frame));
-      else
-	memset (&msymbol, 0, sizeof (msymbol));
-
-      if (msymbol.minsym != NULL
-	  && (BMSYMBOL_VALUE_ADDRESS (msymbol)
-	      > BLOCK_ENTRY_PC (SYMBOL_BLOCK_VALUE (func))))
-	{
-	  /* We also don't know anything about the function besides
-	     its address and name.  */
-	  func = 0;
-	  funname.reset (xstrdup (MSYMBOL_PRINT_NAME (msymbol.minsym)));
-	  *funlang = MSYMBOL_LANGUAGE (msymbol.minsym);
-	}
-      else
     {
       const char *print_name = SYMBOL_PRINT_NAME (func);
 
@@ -1195,7 +1155,6 @@ find_frame_funname (struct frame_info *frame, enum language *funlang,
       if (funname == NULL)
 	funname.reset (xstrdup (print_name));
     }
-    }
   else
     {
       struct bound_minimal_symbol msymbol;

Patch

diff --git a/gdb/stack.c b/gdb/stack.c
index b3d113d3b4..e0a7403ec0 100644
--- a/gdb/stack.c
+++ b/gdb/stack.c
@@ -1135,66 +1135,25 @@  find_frame_funname (struct frame_info *frame, enum language *funlang,
   func = get_frame_function (frame);
   if (func)
     {
-      /* In certain pathological cases, the symtabs give the wrong
-         function (when we are in the first function in a file which
-         is compiled without debugging symbols, the previous function
-         is compiled with debugging symbols, and the "foo.o" symbol
-         that is supposed to tell us where the file with debugging
-         symbols ends has been truncated by ar because it is longer
-         than 15 characters).  This also occurs if the user uses asm()
-         to create a function but not stabs for it (in a file compiled
-         with -g).
-
-         So look in the minimal symbol tables as well, and if it comes
-         up with a larger address for the function use that instead.
-         I don't think this can ever cause any problems; there
-         shouldn't be any minimal symbols in the middle of a function;
-         if this is ever changed many parts of GDB will need to be
-         changed (and we'll create a find_pc_minimal_function or some
-         such).  */
+      const char *print_name = SYMBOL_PRINT_NAME (func);
 
-      struct bound_minimal_symbol msymbol;
-
-      /* Don't attempt to do this for inlined functions, which do not
-	 have a corresponding minimal symbol.  */
-      if (!block_inlined_p (SYMBOL_BLOCK_VALUE (func)))
-	msymbol
-	  = lookup_minimal_symbol_by_pc (get_frame_address_in_block (frame));
-      else
-	memset (&msymbol, 0, sizeof (msymbol));
-
-      if (msymbol.minsym != NULL
-	  && (BMSYMBOL_VALUE_ADDRESS (msymbol)
-	      > BLOCK_ENTRY_PC (SYMBOL_BLOCK_VALUE (func))))
+      *funlang = SYMBOL_LANGUAGE (func);
+      if (funcp)
+	*funcp = func;
+      if (*funlang == language_cplus)
 	{
-	  /* We also don't know anything about the function besides
-	     its address and name.  */
-	  func = 0;
-	  funname.reset (xstrdup (MSYMBOL_PRINT_NAME (msymbol.minsym)));
-	  *funlang = MSYMBOL_LANGUAGE (msymbol.minsym);
+	  /* It seems appropriate to use SYMBOL_PRINT_NAME() here,
+	     to display the demangled name that we already have
+	     stored in the symbol table, but we stored a version
+	     with DMGL_PARAMS turned on, and here we don't want to
+	     display parameters.  So remove the parameters.  */
+	  funname = cp_remove_params (print_name);
 	}
-      else
-	{
-	  const char *print_name = SYMBOL_PRINT_NAME (func);
 
-	  *funlang = SYMBOL_LANGUAGE (func);
-	  if (funcp)
-	    *funcp = func;
-	  if (*funlang == language_cplus)
-	    {
-	      /* It seems appropriate to use SYMBOL_PRINT_NAME() here,
-		 to display the demangled name that we already have
-		 stored in the symbol table, but we stored a version
-		 with DMGL_PARAMS turned on, and here we don't want to
-		 display parameters.  So remove the parameters.  */
-	      funname = cp_remove_params (print_name);
-	    }
-
-	  /* If we didn't hit the C++ case above, set *funname
-	     here.  */
-	  if (funname == NULL)
-	    funname.reset (xstrdup (print_name));
-	}
+      /* If we didn't hit the C++ case above, set *funname
+	 here.  */
+      if (funname == NULL)
+	funname.reset (xstrdup (print_name));
     }
   else
     {