Hi Alexey,
sorry for the late reply, I needed to learn the crash-gdb interaction and
how to review such a huge patch as this.  We appreciate your patience and
understanding.
I've almost read the patch, on the whole, it is absolutely excellent!
I think it's going a right way.
There are several comments including slight things:
- Is it possible to separate the fixes only in crash (outside gdb-10.1.patch)
and old patches removal from this 1/2 patch? i.e.
  - fix for "'bt' command often emits reduced output"
  - fix for "lack of information about struct members" and
"unionprint"
  - removal of gdb-7.6.patch and gdb-7.6-proc_service.h.patch
This would help us understand what issue a bunch of changes fixes, and
we can read it easier.  I know it's better not to split the gdb-10.1.patch.
- Build error on architectures except for x86_64:
/usr/bin/ld: ../../crashlib.a(gdb_interface.o): in function `crash_get_nr_cpus':
/home/travis/build/k-hagio/crash/gdb_interface.c:1074: undefined reference to
`sadump_get_nr_cpus'
/usr/bin/ld: /home/travis/build/k-hagio/crash/gdb_interface.c:1076: undefined reference to
`diskdump_get_nr_cpus'
/usr/bin/ld: /home/travis/build/k-hagio/crash/gdb_interface.c:1078: undefined reference to
`kdump_get_nr_cpus'
collect2: error: ld returned 1 exit status
make[4]: *** [Makefile:1872: gdb] Error 1
make[3]: *** [Makefile:10072: all-gdb] Error 2
make[2]: *** [Makefile:860: all] Error 2
crash build failed
make[1]: *** [Makefile:239: gdb_merge] Error 1
make: *** [Makefile:314: warn] Error 2
The command "make warn" exited with 2.
ref. 
https://travis-ci.org/github/k-hagio/crash/builds/759444845
- "make target=x86" on an x86_64 machine also fails with additional errors:
$ make target=x86
...
ar: creating crashlib.a
  CXXLD  gdb
/usr/bin/ld: skipping incompatible ./../zlib/libz.a when searching for -lz
/usr/bin/ld: skipping incompatible ./../zlib/libz.a when searching for -lz
/usr/bin/ld: i386 architecture of input file `../../crashlib.a(main.o)' is
incompatible with i386:x86-64 output
/usr/bin/ld: i386 architecture of input file `../../crashlib.a(tools.o)' is
incompatible with i386:x86-64 output
/usr/bin/ld: i386 architecture of input file `../../crashlib.a(global_data.o)' is
incompatible with i386:x86-64 output
...
../../crashlib.a(tools.o): In function `eval_common':
/home/k-hagio/crash/x86-gdb10/tools.c:3012: undefined reference to `__udivdi3'
/home/k-hagio/crash/x86-gdb10/tools.c:3015: undefined reference to `__umoddi3'
...
decNumber.c:(.text+0x79a7): undefined reference to `__udivdi3'
collect2: error: ld returned 1 exit status
make[3]: *** [gdb] Error 1
make[2]: *** [rebuild] Error 2
make[1]: *** [gdb_merge] Error 2
make: *** [all] Error 2
- "p" does not print the address of "linux_banner" for some vmcores
(relatively old kernels? like RHEL7).
--- crash-master.log
+++ crash-gdb10.1.log
crash> p linux_banner
-linux_banner = $1 = 0xffffffff816bc100 <linux_banner> "Linux version ...
+linux_banner = $1 = "Linux version ...
- "whatis" options print reduced/duplicated results:
crash> whatis -r 512
 SIZE  TYPE
- 512  _legacy_mbr  <<-- dropped
  512  i387_fxsave_struct
  512  netns_ipv4
- 512  sgi_disklabel
  512  user_i387_struct
crash> whatis -m mm_struct
 SIZE  TYPE
   16  tlb_state
-  24  flush_tlb_info  <<-- dropped
-  24  ftrace_raw_xen_mmu_pgd
   24  futex_key
...
  216  vm_area_struct
  256  linux_binprm
 2752  rq
+2752  rq  <<-- duplicated
+2752  rq
+2752  rq
+2752  rq
 4048  task_struct
-8296  numa_maps_private
 +--- gdb-10.1/gdb/main.c.orig
 ++++ gdb-10.1/gdb/main.c
 +@@ -929,8 +944,12 @@ captured_main_1 (struct captured_main_args
 +          catch_command_errors returns non-zero on success!  */
 +       if (catch_command_errors (exec_file_attach, execarg,
 + 				!batch_flag, RETURN_MASK_ALL))
 ++#ifdef CRASH_MERGE
 ++        catch_command_errors (symbol_file_add_main, symarg, 0, RETURN_MASK_ALL);
 ++#else
 + 	catch_command_errors (symbol_file_add_main, symarg,
 + 			      !batch_flag, RETURN_MASK_ALL);
 ++#endif
 +     }
 +   else
 +     { 
- This is a dropped hunk, but without this, "crash -s" also prints the
following message:
# crash -s
Reading symbols from /usr/lib/debug/lib/modules/3.10.0-1127.el7.x86_64/vmlinux...
crash>
I'm ok with this message without the "-s" option, but it would be
preferable
to print nothing with the option if there is no warning.
 +@@ -992,8 +1011,12 @@ captured_main (void *data)
 + 	{
 + 	  auto_load_local_gdbinit_loaded = 1;
 +
 ++#ifdef CRASH_MERGE
 ++          catch_command_errors (source_script, local_gdbinit, -1, RETURN_MASK_ALL);
 ++#else
 + 	  catch_command_errors (source_script, local_gdbinit, 0,
 + 				RETURN_MASK_ALL);
 ++#endif
 + 	}
 +     }
 +
 +@@ -1039,6 +1062,12 @@ captured_main (void *data)
 +   while (1)
 +     {
 +       catch_errors (captured_command_loop, 0, "", RETURN_MASK_ALL);
 ++#ifdef CRASH_MERGE
 ++      {
 ++        int console(char *, ...);
 ++        console("<CAPTURED_MAIN WHILE LOOP>\n");
 ++      }
 ++#endif
 +     }
 +   /* No exit -- exit is through quit_command.  */
 + } 
- Why were the two hunks dropped?  Is it possible not to drop?
 ++static void
 ++gdb_delete_symbol_file(struct gnu_request *req)
 ++{
 ++	for (objfile *objfile : current_program_space->objfiles ()) {
 ++                if (STREQ(objfile_name(objfile), req->name) ||
 ++		    same_file((char *)objfile_name(objfile), req->name)) {
 ++			break;
 ++                }
 ++        } 
- This does not delete the symbol file, so the symbols remain even after
"mod -d" command.
 ++static void
 ++dump_enum(struct type *type, struct gnu_request *req)
 ++{
 ++	register int i;
 ++	int len;
 ++	int lastval; 
- The "lastval" variable should be "long long"?
  #define DUMP_EMPTY_FILE     0x8
  #define DUMP_FILE_NRPAGES  0x10
 -#endif  /* !GDB_COMMON */
  int same_file(char *, char *);
 +#endif  /* !GDB_COMMON */
  #ifndef GDB_COMMON
  int cleanup_memory_driver(void); 
- We can remove this #endif and #ifndef?
 ++#ifdef CRASH_MERGE
 ++extern "C" int gdb_main_entry(int, char **);
 ++extern void replace_ui_file_FILE(struct ui_file *, FILE *); 
- We don't have the replace_ui_file_FILE() any more?
 +int crash_get_nr_cpus(void)
 +{
 +	if (SADUMP_DUMPFILE())
 +		return sadump_get_nr_cpus();
 +	else if (DISKDUMP_DUMPFILE())
 +		return diskdump_get_nr_cpus();
 +	else if (KDUMP_DUMPFILE())
 +		return kdump_get_nr_cpus();
 +	else if (VMSS_DUMPFILE())
 +		return vmware_vmss_get_nr_cpus(); 
- Seems diskdump_get_nr_cpus() and kdump_get_nr_cpus() works only with
QEMU memory dumps and return 0 for normal vmcores.  This causes crash
to fail with the 2/2 patch?
- The gdb-10.1.patch does not have a shell script at the head of it, once
it's modified, "make" prints "gdb-10.1.patch: line 2: ---: command not
found"
and so on.
$ make warn
TARGET: X86_64
 CRASH: 7.2.9++
   GDB: 10.1
+ diff -aurp -X diff_exclude gdb-10.1.orig/gdb/cli/cli-cmds.c gdb-10.1/gdb/cli/cli-cmds.c
diff: diff_exclude: No such file or directory
+ --- gdb-10.1.orig/gdb/cli/cli-cmds.c 2020-10-23 21:23:02.000000000 -0700
gdb-10.1.patch: line 2: ---: command not found
+ +++ gdb-10.1/gdb/cli/cli-cmds.c 2020-11-10 13:06:56.423569114 -0800
gdb-10.1.patch: line 3: +++: command not found
gdb-10.1.patch: line 4: syntax error near unexpected token `('
gdb-10.1.patch: line 4: `@@ -435,6 +435,11 @@ complete_command (const char *arg, int
f'
patching file gdb-10.1/gdb/cli/cli-cmds.c
Reversed (or previously applied) patch detected!  Skipping patch.
4 out of 4 hunks ignored
...
- Compilation warnings.
symtab.c: In function ‘void gdb_get_line_number(gnu_request*)’:
symtab.c:7073:17: warning: variable ‘sym’ set but not used [-Wunused-but-set-variable]
  struct symbol *sym;
                 ^
  CXX    gcore.o
symtab.c: In function ‘void gdb_get_datatype(gnu_request*)’:
symtab.c:7137:51: warning: deprecated conversion from string constant to ‘char*’
[-Wwrite-strings]
   console("gdb_get_datatype [%s] (a)\n", req->name);
                                                   ^
  CXX    gdb-demangle.o
symtab.c:7163:51: warning: deprecated conversion from string constant to ‘char*’
[-Wwrite-strings]
   console("gdb_get_datatype [%s] (b)\n", req->name);
                                                   ^
symtab.c:7172:57: warning: deprecated conversion from string constant to ‘char*’
[-Wwrite-strings]
           console("expr->elts[0].opcode: OP_VAR_VALUE\n");
                                                         ^
  CXX    gdb_bfd.o
symtab.c:7191:52: warning: deprecated conversion from string constant to ‘char*’
[-Wwrite-strings]
           console("expr->elts[0].opcode: OP_TYPE\n");
                                                    ^
symtab.c:7224:31: warning: deprecated conversion from string constant to ‘char*’
[-Wwrite-strings]
     expr.get()->elts[0].opcode);
                               ^
symtab.c: In function ‘void eval_enum(type*, gnu_request*)’:
symtab.c:7285:17: warning: deprecated conversion from string constant to ‘char*’
[-Wwrite-strings]
    req->tagname = "(unknown)";
                 ^
symtab.c: In function ‘void get_member_data(gnu_request*, type*, long int, int)’:
symtab.c:7315:42: warning: deprecated conversion from string constant to ‘char*’
[-Wwrite-strings]
     req->name, req->member, type, newtype);
                                          ^
symtab.c: In function ‘void gdb_command_exists(gnu_request*)’:
symtab.c:7355:43: warning: variable ‘c’ set but not used [-Wunused-but-set-variable]
         register struct cmd_list_element *c;
Thanks,
Kazu
-----Original Message-----
 Fully redone gdb-7.6.patch to gdb-10.1.patch to keep all
 functionality. Changes which were dropped are saved in
 dropped-gdb-7.6-to-10.1.patch
 
 Main difference between gdb-7.6 and gdb-10.1 is the last
 one was rewritten in C++.
 I continue to keep crash code in C. Mark transition
 functions as extern "C" to resolve linking issues.
 
 Eliminated error_hook() and SJLJ while running in C++ code
 (after gdb_command_funnel()) use try-catch mechanism instead.
 
 request_types() was redone to do not call
 GNU_GET_NEXT_DATATYPE multiple times but single usage of
 GNU_ITERATE_DATATYPES with proper callback instead.
 Complete iteration happens on C++ side now.
 Removed "struct global_iterator" from request structure,
 but added several fields (including callback pointer) to
 be able to perform iteration on C++ side.
 
 Type of "linux_banner" symbol is reported as 'D' by new
 gdb as its section ".rodata" marked as writable in vmlinux.
 
 BFD API has changed.
 
 deprecated_command_loop_hook got deprecated. So, call crash
 main_loop() directly from gdb captured_main().
 
 Added symbol file (vmlinux) rebase in gdb by kaslr_offset.
 by using new function: objfile_rebase().
 As result, we do not need kernel symbol patching as well as
 bait_and_switch hook anymore.
 
 Added crash_target for gdb to provide target operations
 such as xfer_partial to read and write crash dump memory.
 Removed previously used hooks for that in target.c.
 Keep crash_target.c as a file in crash folder instead of
 in gdb-10.1.patch for easier development and history
 tracking.
 crash_target can be enhanced in future to provide access
 to CPU registers, so backtrace and frame related commands
 from gdb can be used.
 
 Removed gdb-7.6-proc_service.h.patch is not required as
 gdb-10.1 already has this change.
 
 Extra: add VMware copyright to the version info.
 
 TODO:
 1) gdb-10.1-ppc64le-support.patch has to be updated with
 following commits.
 2) deprecate #if defined(GDB_X_Y) code as crash really
 supports only the latest gdb (only one patch).
 3) move gdb_funnel_command() and subfunctions to separate
 file, similar to crash_target.c
 4) remove legacy kernel patching and bait_and_switch hook.
 
 Signed-off-by: Alexey Makhalov <amakhalov(a)vmware.com>
 ---
  Makefile                                           |   11 +-
  configure.c                                        |   20 +-
  crash_target.c                                     |  104 +
  defs.h                                             |   35 +-
  dropped-gdb-7.6-to-10.1.patch                      |  303 +++
  ...support.patch => gdb-10.1-ppc64le-support.patch |    0
  gdb-10.1.patch                                     | 1577 ++++++++++++
  gdb-7.6-proc_service.h.patch                       |   67 -
  gdb-7.6.patch                                      | 2503 --------------------
  gdb_interface.c                                    |   85 +-
  help.c                                             |    1 +
  kernel.c                                           |    2 +-
  main.c                                             |    1 -
  symbols.c                                          |  125 +-
  x86_64.c                                           |   14 +-
  15 files changed, 2141 insertions(+), 2707 deletions(-)
  create mode 100644 crash_target.c
  create mode 100644 dropped-gdb-7.6-to-10.1.patch
  rename gdb-7.6-ppc64le-support.patch => gdb-10.1-ppc64le-support.patch (100%)
  create mode 100644 gdb-10.1.patch
  delete mode 100644 gdb-7.6-proc_service.h.patch
  delete mode 100644 gdb-7.6.patch