Re: [PATCH v2][GDB][ARM]: gdb cannot step across CMSE secure entry function code.

[Alan Hayward <Alan dot Hayward at arm dot com>]

> On 19 Jul 2019, at 13:23, Srinath Parvathaneni <Srinath.Parvathaneni@arm.com> wrote:
> 
> Hello Alan, Simon and Tom.
> 
> Thank you very much for reviewing my first patch to GDB and providing me your feedback.
> https://sourceware.org/ml/gdb-patches/2019-07/msg00392.html
> 
> I have incorporating all your review comments and sending a new patch here.
> 
> Hello,
> 
> GDB is not able to execute "step" command on function calls of Armv8-M cmse secure entry functions.
> Everytime GNU linker come across definition of any cmse secure entry function in object file(s),
> it creates two new instructions secure gateway (sg) and original branch destination (b.w),
> place those two instructions in ".gnu.sgstubs" section of executable.
> Any function calls to these cmse secure entry functions is re-directed through secure gateway (sg)
> present in ".gnu.sgstubs" section.
> 
> Example:
> Following is a function call to cmse secure entry function "foo":
>        ...
>        bl xxxx <foo>   --->(a)
>        ...
>        <foo>
>        xxxx: push    {r7, lr}
> 
> GNU linker on finding out "foo" is a cmse secure entry function, created sg and b.w instructions and
> place them in ".gnu.sgstubs" section (marked by c).
> 
> The "bl" instruction (marked by a) which is a call to cmse secure entry function is modified by GNU linker
> (as marked by b) and call flow is re-directly through secure gateway (sg) in ".gnu.sgstubs" section.
>       ...
>       bl yyyy <foo>  ---> (b)
>       ...
>       section .gnu.sgstubs: ---> (c)
>       yyyy <foo>
>       yyyy: sg   // secure gateway
> 	     b.w xxxx <__acle_se_foo>  // original_branch_dest
>       ...
>       0000xxxx <__acle_se_foo>
>       xxxx: push    {r7, lr} ---> (d)
> 
> On invoking GDB, when the control is at "b" and we pass "step" command, the pc returns "yyyy"
> (sg address) which is a trampoline and which does not exist in source code. So GDB jumps
> to next line without jumping to "__acle_se_foo" (marked by d).
> 
> The above details are published on the Arm website [1], please refer to section 5.4 (Entry functions)
> and section 3.4.4 (C level development flow of secure code).
> 
> [1] https://developer.arm.com/architectures/cpu-architecture/m-profile/docs/ecm0359818/latest/armv8-m-security-extensions-requirements-on-development-tools-engineering-specification
> 
> This patch fixes above problem by returning target pc "xxxx" to GDB on executing "step"
> command at "b", so that the control jumps to "__acle_se_foo" (marked by d).
> 
> This patch is tested by debugging the CMSE executable using GDB on Aarch32 box.
> Regression tested for armv7hl-redhat-linux-gnueabi and found no regressions.
> 
> Ok for master? If ok, could someone commit this on my behalf,
> I don't have the commit rights.

LGTM.
I tried the patch, and it works for me.

I’ll give it a few days for someone else to comment, and if not, then I'll commit
early next week.


Thanks!
Alan.



> 
> gdb/ChangeLog:
> 
> 2019-07-19  Srinath Parvathaneni  <srinath.parvathaneni@arm.com>
> 
> 	* gdb/arm-tdep.c (arm_skip_cmse_entry):New function. When gdb
> 	encounters a "step" command on cmse secure entry function (eg:func),
> 	this function return an address of "__acle_se_<func>" to PC instead
> 	of secure gateaway (sg) address which is present in ".gnu.sgstubs"
> 	section.
> 	(arm_is_sgstubs_section):New function. To check the current section is
> 	".gnu.sgstubs".
> 	(arm_skip_stub):Add call to arm_skip_cmse_entry function.
> 	* gdb/arm-tdep.h (arm_is_sgstubs_section):New function declaration.
> 
> gdb/testsuite/ChangeLog:
> 
> 2019-07-19  Srinath Parvathaneni  <srinath.parvathaneni@arm.com>
> 
> 	* gdb.arch/arm-cmse-sgstubs.c: New test.
> 	* gdb.arch/arm-cmse-sgstubs.exp: New file.
> 
> 
> 
> ###############     Attachment also inlined for ease of reply    ###############
> 
> 
> diff --git a/gdb/arm-tdep.h b/gdb/arm-tdep.h
> index 23dd40ea8beb1b00289a4cd4e65647399d351580..9482e765a7fc5bb58676096f6b879eae2a7c858e 100644
> --- a/gdb/arm-tdep.h
> +++ b/gdb/arm-tdep.h
> @@ -259,6 +259,7 @@ ULONGEST arm_get_next_pcs_read_memory_unsigned_integer (CORE_ADDR memaddr,
> 
> CORE_ADDR arm_get_next_pcs_addr_bits_remove (struct arm_get_next_pcs *self,
> 					     CORE_ADDR val);
> +bool arm_is_sgstubs_section (struct obj_section *);
> 
> int arm_get_next_pcs_is_thumb (struct arm_get_next_pcs *self);
> 
> diff --git a/gdb/arm-tdep.c b/gdb/arm-tdep.c
> index d244707210628ab045f677c0cbad3d8b0c6d6269..e3b7ab6f096eb91da067d772b8798ffd0737e3d6 100644
> --- a/gdb/arm-tdep.c
> +++ b/gdb/arm-tdep.c
> @@ -8211,6 +8211,53 @@ arm_get_longjmp_target (struct frame_info *frame, CORE_ADDR *pc)
>   *pc = extract_unsigned_integer (buf, INT_REGISTER_SIZE, byte_order);
>   return 1;
> }
> +/* A call to cmse secure entry function "foo" at "a" is modified by
> +     GNU ld as "b".
> +     a) bl xxxx <foo>
> +
> +     <foo>
> +     xxxx:
> +
> +     b) bl yyyy <__acle_se_foo>
> +
> +     section .gnu.sgstubs:
> +     <foo>
> +     yyyy: sg   // secure gateway
> +	   b.w xxxx <__acle_se_foo>  // original_branch_dest
> +
> +     <__acle_se_foo>
> +     xxxx:
> +
> +  When the control at "b", the pc contains "yyyy" (sg address) which is a
> +  trampoline and does not exist in source code.  This function returns the
> +  target pc "xxxx".  For more details please refer to section 5.4
> +  (Entry functions) and section 3.4.4 (C level development flow of secure code)
> +  of "armv8-m-security-extensions-requirements-on-development-tools-engineering-specification"
> +  document on www.developer.arm.com.  */
> +
> +static CORE_ADDR
> +arm_skip_cmse_entry (CORE_ADDR pc, const char *name, struct objfile *objfile)
> +{
> +  struct bound_minimal_symbol minsym;
> +  int target_len = strlen (name) + strlen ("__acle_se_") + 1;
> +  char *target_name  = (char *) alloca (target_len);
> +  xsnprintf (target_name, target_len, "%s%s", "__acle_se_",name);
> +  minsym = lookup_minimal_symbol (target_name, NULL, objfile);
> +  if (minsym.minsym != nullptr)
> +    return BMSYMBOL_VALUE_ADDRESS (minsym);
> +  return 0;
> +}
> +
> +/* Return true when sec points to ".gnu.sgstubs" section.  */
> +bool
> +arm_is_sgstubs_section (struct obj_section *sec)
> +{
> + if (sec != nullptr && sec->the_bfd_section != nullptr
> +     && sec->the_bfd_section->name != nullptr
> +     && streq (sec->the_bfd_section->name,".gnu.sgstubs"))
> +   return true;
> + return false;
> +}
> 
> /* Recognize GCC and GNU ld's trampolines.  If we are in a trampoline,
>    return the target PC.  Otherwise return 0.  */
> @@ -8221,6 +8268,7 @@ arm_skip_stub (struct frame_info *frame, CORE_ADDR pc)
>   const char *name;
>   int namelen;
>   CORE_ADDR start_addr;
> +  struct obj_section *section;
> 
>   /* Find the starting address and name of the function containing the PC.  */
>   if (find_pc_partial_function (pc, &name, &start_addr, NULL) == 0)
> @@ -8290,6 +8338,10 @@ arm_skip_stub (struct frame_info *frame, CORE_ADDR pc)
> 	return 0;
>     }
> 
> +  section = find_pc_section (pc);
> +  /* checks whether address pc holds belows to ".gnu.sgstubs" section.  */
> +  if (arm_is_sgstubs_section (section))
> +    return arm_skip_cmse_entry (pc, name, section->objfile);
>   return 0;			/* not a stub */
> }
> 
> diff --git a/gdb/testsuite/gdb.arch/arm-cmse-sgstubs.c b/gdb/testsuite/gdb.arch/arm-cmse-sgstubs.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..7f3b40f20c67abfdd2410614e7ee29ae77d37966
> --- /dev/null
> +++ b/gdb/testsuite/gdb.arch/arm-cmse-sgstubs.c
> @@ -0,0 +1,29 @@
> +#include <stdio.h>
> +extern void func();
> +void __acle_se_func ()
> +{
> +  printf("__acle_se_func\n");
> +}
> +
> +/* The following code is written using asm so that the instructions in function
> + * "func" will be placed in .gnu.sgstubs section of the executable.  */
> +asm ("\t.section .gnu.sgstubs,\"ax\",%progbits\n"
> +     "\t.global func\n"
> +     "\t.type func, %function\n"
> +     "func:\n"
> +     "\tnop @sg\n"
> +     "\tb __acle_se_func @b.w");
> +
> +void fun1 ()
> +{
> +  printf("In fun1\n");
> +}
> +
> +int main (void)
> +{
> +  func();
> +  fun1();
> +  __acle_se_func();
> +  func();
> +  return 0;
> +}
> diff --git a/gdb/testsuite/gdb.arch/arm-cmse-sgstubs.exp b/gdb/testsuite/gdb.arch/arm-cmse-sgstubs.exp
> new file mode 100644
> index 0000000000000000000000000000000000000000..3416e887d9ebe5ebc52336eff15ba83a6d16df21
> --- /dev/null
> +++ b/gdb/testsuite/gdb.arch/arm-cmse-sgstubs.exp
> @@ -0,0 +1,60 @@
> +# Copyright 2019 Free Software Foundation, Inc.
> +
> +# This program is free software; you can redistribute it and/or modify
> +# it under the terms of the GNU General Public License as published by
> +# the Free Software Foundation; either version 3 of the License, or
> +# (at your option) any later version.
> +#
> +# This program is distributed in the hope that it will be useful,
> +# but WITHOUT ANY WARRANTY; without even the implied warranty of
> +# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +# GNU General Public License for more details.
> +#
> +# You should have received a copy of the GNU General Public License
> +# along with this program.  If not, see <http://www.gnu.org/licenses/>.
> +
> +# This file is part of the gdb testsuite.
> +
> +if { ![istarget "arm*-*-*"]} {
> +     return 1
> +}
> +
> +standard_testfile
> +if { [prepare_for_testing "failed to prepare" $testfile $srcfile ]} {
> +    return -1
> +}
> +
> +if ![runto_main] {
> +   untested "could not run to main"
> +   return -1
> +}
> +
> +set test "branch to func from main"
> +gdb_test "si" "0x.*" "$test"
> +
> +set test "next instruction in func"
> +gdb_test "ni" "0x.*" "$test"
> +
> +set test "branch to __acle_se_func from func"
> +gdb_test "ni" "__acle_se_func ().*" "${test}"
> +
> +set test "next in __acle_se_func function"
> +gdb_test "next" "5	  .*" "$test"
> +
> +set test "next in __acle_se_func function outputs __acle_se_func"
> +gdb_test "next" "__acle_se_func.*" "$test"
> +
> +set test "next in __acle_se_func function controls returns to main"
> +gdb_test "next" "main ().*" "$test"
> +
> +set test "next in main outputs In fun1"
> +gdb_test "next" "In fun1.*" "$test"
> +
> +set test "next in main outputs __acle_se_func"
> +gdb_test "next" "__acle_se_func.*" "$test"
> +
> +set test "control jumps to __acle_se_func from main via func"
> +gdb_test "step" "__acle_se_func ().*" "${test}"
> +
> +set test "next in __acle_se_func function via func"
> +gdb_test "next" "__acle_se_func.*" "$test"
> 
> <rb11415.patch>