[PATCH] Fix reading .dwp files without .debug_tu_index

[Alexander Shaposhnikov <alexander.v.shaposhnikov@gmail.com>]

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This patch fixes segmentation fault (due to dereferencing of a null pointer)
in dwarf2read.c in the code dwp_file->cus->version != dwp_file->tus->version
by adding defensive checks similarly to how it's already done
at the lines 11208 - 11215 (in the same file dwarf2read.c).
The issue can be reproduced with dwp packages built by llvm-dwp utility
(from the current upstream) since at the moment llvm-dwp doesn't create
.debug_tu_index section, thus dwp_file->tus will be null.

Test plan:

BUILD:
main.cpp:
int f() {
  int x = 0;
  int y = 1;
  return x + y;
}
g++ -fPIC -gsplit-dwarf -g -O0 main.cpp -o main.exe
llvm-dwp main.dwo -o main.exe.dwp
# This step is a workaround to a separate issue (unrelated to this patch).
# At the moment llvm tools & clang put .strtab section first (its index is
1),
# while gcc/gold/binutils put it at the end.
# Unfortunately gdb (in the code reading dwps) appears to depend on the
order
# of sections, to workaround this (to reproduce the issue which this patch
# aims to address) we use objcopy to do the following trick:
# if one asks to remove .strtab objcopy will do that but at the same time
# it will create a new .shstrtab section at the end.
objcopy --remove-section .strtab main.exe.dwp
RUN:
gdb main.exe
One can observe that now there is no crash and debugging functionality
works as expected (setting breakpoints, printing local variable, exploring
the stack).

gdb/ChangeLog:
yyyy-mm-dd  Alexander Shaposhnikov <alexander.v.shaposhnikov@gmail.com>

* dwarf2read.c: Fix segmentation fault on reading

  .dwp files without .debug_tu_index section.

Patch:

diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c

index b1914cf876..547e3f034e 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -11185,7 +11185,8 @@ open_and_init_dwp_file (void)
   dwp_file->tus = create_dwp_hash_table (dwp_file, 1);

   /* The DWP file version is stored in the hash table.  Oh well.  */
-  if (dwp_file->cus->version != dwp_file->tus->version)
+  if (dwp_file->cus && dwp_file->tus
+      && dwp_file->cus->version != dwp_file->tus->version)
     {
       /* Technically speaking, we should try to limp along, but this is
 	 pretty bizarre.  We use pulongest here because that's the established
@@ -11195,7 +11196,7 @@ open_and_init_dwp_file (void)
 	     pulongest (dwp_file->cus->version),
 	     pulongest (dwp_file->tus->version), dwp_name.c_str ());
     }
-  dwp_file->version = dwp_file->cus->version;
+  dwp_file->version = dwp_file->cus ? dwp_file->cus->version : 0;

   if (dwp_file->version == 2)
     bfd_map_over_sections (dwp_file->dbfd, dwarf2_locate_v2_dwp_sections,


Kind regards,
Alexander Shaposhnikov