gdb: support for eBPF

This patch adds basic support for the eBPF target: tdep and build
machinery.  The accompanying simulator is introduced in subsequent
patches.

gdb/ChangeLog:

2020-08-04  Weimin Pan <weimin.pan@oracle.com>
	    Jose E. Marchesi  <jose.marchesi@oracle.com>

	* configure.tgt: Add entry for bpf-*-*.
	* Makefile.in (ALL_TARGET_OBS): Add bpf-tdep.o
	(ALLDEPFILES): Add bpf-tdep.c.
	* bpf-tdep.c: New file.
	* MAINTAINERS: Add bpf target and maintainer.

gdb/doc/ChangeLog:

2020-08-04  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* gdb.texinfo (Contributors): Add information for the eBPF
	support.
	(BPF): New section.

1 files changed, 386 insertions, 0 deletions

diff --git a/gdb/bpf-tdep.c b/gdb/bpf-tdep.c
new file mode 100644
index 0000000..4854325
--- /dev/null
+++ b/gdb/bpf-tdep.c
@@ -0,0 +1,386 @@
+/* Target-dependent code for BPF.
+
+   Copyright (C) 2020 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   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/>.  */
+
+#include "defs.h"
+#include "arch-utils.h"
+#include "dis-asm.h"
+#include "frame.h"
+#include "frame-unwind.h"
+#include "trad-frame.h"
+#include "symtab.h"
+#include "value.h"
+#include "gdbcmd.h"
+#include "breakpoint.h"
+#include "inferior.h"
+#include "regcache.h"
+#include "target.h"
+#include "dwarf2/frame.h"
+#include "osabi.h"
+#include "target-descriptions.h"
+#include "remote.h"
+
+
+/* eBPF registers.  */
+
+enum bpf_regnum
+{
+  BPF_R0_REGNUM,		/* return value */
+  BPF_R1_REGNUM,
+  BPF_R2_REGNUM,
+  BPF_R3_REGNUM,
+  BPF_R4_REGNUM,
+  BPF_R5_REGNUM,
+  BPF_R6_REGNUM,
+  BPF_R7_REGNUM,
+  BPF_R8_REGNUM,
+  BPF_R9_REGNUM,
+  BPF_R10_REGNUM,		/* sp */
+  BPF_PC_REGNUM,
+};
+
+#define BPF_NUM_REGS	(BPF_PC_REGNUM + 1)
+
+/* Target-dependent structure in gdbarch.  */
+struct gdbarch_tdep
+{
+};
+
+
+/* Internal debugging facilities.  */
+
+/* When this is set to non-zero debugging information will be
+   printed.  */
+
+static unsigned int bpf_debug_flag = 0;
+
+/* The show callback for 'show debug bpf'.  */
+
+static void
+show_bpf_debug (struct ui_file *file, int from_tty,
+	        struct cmd_list_element *c, const char *value)
+{
+  fprintf_filtered (file, _("Debugging of BPF is %s.\n"), value);
+}
+
+
+/* BPF registers.  */
+
+static const char *bpf_register_names[] =
+{
+  "r0",   "r1",  "r2",    "r3",   "r4",   "r5",   "r6",   "r7",
+  "r8",   "r9",  "r10",   "pc"
+};
+
+/* Return the name of register REGNUM.  */
+
+static const char *
+bpf_register_name (struct gdbarch *gdbarch, int reg)
+{
+  if (reg >= 0 && reg < BPF_NUM_REGS)
+    return bpf_register_names[reg];
+  return NULL;
+}
+
+/* Return the GDB type of register REGNUM.  */
+
+static struct type *
+bpf_register_type (struct gdbarch *gdbarch, int reg)
+{
+  if (reg == BPF_R10_REGNUM)
+    return builtin_type (gdbarch)->builtin_data_ptr;
+  else if (reg == BPF_PC_REGNUM)
+    return builtin_type (gdbarch)->builtin_func_ptr;
+  return builtin_type (gdbarch)->builtin_int64;
+}
+
+/* Return the GDB register number corresponding to DWARF's REG.  */
+
+static int
+bpf_dwarf2_reg_to_regnum (struct gdbarch *gdbarch, int reg)
+{
+  if (reg >= 0 && reg < BPF_NUM_REGS)
+    return reg;
+  return -1;
+}
+
+/* Implement the "print_insn" gdbarch method.  */
+
+static int
+bpf_gdb_print_insn (bfd_vma memaddr, disassemble_info *info)
+{
+  info->symbols = NULL;
+  return default_print_insn (memaddr, info);
+}
+
+
+/* Return PC of first real instruction of the function starting at
+   START_PC.  */
+
+static CORE_ADDR
+bpf_skip_prologue (struct gdbarch *gdbarch, CORE_ADDR start_pc)
+{
+  fprintf_unfiltered (gdb_stdlog,
+		      "Skipping prologue: start_pc=%s\n",
+		      paddress (gdbarch, start_pc));
+  /* XXX: to be completed.  */
+  return start_pc + 0;
+}
+
+
+/* Frame unwinder.
+
+   XXX it is not clear how to unwind in eBPF, since the stack is not
+   guaranteed to be contiguous, and therefore no relative stack
+   addressing can be done in the callee in order to access the
+   caller's stack frame.  To explore with xBPF, which will relax this
+   restriction.  */
+
+/* Given THIS_FRAME, return its ID.  */
+
+static void
+bpf_frame_this_id (struct frame_info *this_frame,
+		   void **this_prologue_cache,
+		   struct frame_id *this_id)
+{
+  /* Note that THIS_ID defaults to the outermost frame if we don't set
+     anything here.  See frame.c:compute_frame_id.  */
+}
+
+/* Return the reason why we can't unwind past THIS_FRAME.  */
+
+static enum unwind_stop_reason
+bpf_frame_unwind_stop_reason (struct frame_info *this_frame,
+			      void **this_cache)
+{
+  return UNWIND_OUTERMOST;
+}
+
+/* Ask THIS_FRAME to unwind its register.  */
+
+static struct value *
+bpf_frame_prev_register (struct frame_info *this_frame,
+			 void **this_prologue_cache, int regnum)
+{
+  return frame_unwind_got_register (this_frame, regnum, regnum);
+}
+
+/* Frame unwinder machinery for BPF.  */
+
+static const struct frame_unwind bpf_frame_unwind =
+{
+  NORMAL_FRAME,
+  bpf_frame_unwind_stop_reason,
+  bpf_frame_this_id,
+  bpf_frame_prev_register,
+  NULL,
+  default_frame_sniffer
+};
+
+
+/* Breakpoints.  */
+
+/* Enum describing the different kinds of breakpoints.  We currently
+   just support one, implemented by the brkpt xbpf instruction.   */
+
+enum bpf_breakpoint_kinds
+{
+  BPF_BP_KIND_BRKPT = 0,
+};
+
+/* Implement the breakpoint_kind_from_pc gdbarch method.  */
+
+static int
+bpf_breakpoint_kind_from_pc (struct gdbarch *gdbarch, CORE_ADDR *start_pc)
+{
+  /* We support just one kind of breakpoint.  */
+  return BPF_BP_KIND_BRKPT;
+}
+
+/* Implement the sw_breakpoint_from_kind gdbarch method.  */
+
+static const gdb_byte *
+bpf_sw_breakpoint_from_kind (struct gdbarch *gdbarch, int kind, int *size)
+{
+  static unsigned char brkpt_insn[]
+    = {0x8c, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00};
+
+  switch (kind)
+    {
+    case BPF_BP_KIND_BRKPT:
+      *size = 8;
+      return brkpt_insn;
+    default:
+      gdb_assert_not_reached ("unexpected BPF breakpoint kind");
+    }
+}
+
+
+/* Assuming THIS_FRAME is a dummy frame, return its frame ID.  */
+
+static struct frame_id
+bpf_dummy_id (struct gdbarch *gdbarch, struct frame_info *this_frame)
+{
+  CORE_ADDR sp = get_frame_register_unsigned (this_frame,
+					      gdbarch_sp_regnum (gdbarch));
+  return frame_id_build (sp, get_frame_pc (this_frame));
+}
+
+/* Implement the push dummy call gdbarch callback.  */
+
+static CORE_ADDR
+bpf_push_dummy_call (struct gdbarch *gdbarch, struct value *function,
+		     struct regcache *regcache, CORE_ADDR bp_addr,
+		     int nargs, struct value **args, CORE_ADDR sp,
+		     function_call_return_method return_method,
+		     CORE_ADDR struct_addr)
+{
+  fprintf_unfiltered (gdb_stdlog, "Pushing dummy call: sp=%s\n",
+		      paddress (gdbarch, sp));
+  /* XXX writeme  */
+  return sp;
+}
+
+/* Extract a function return value of TYPE from REGCACHE,
+   and copy it into VALBUF.  */
+
+static void
+bpf_extract_return_value (struct type *type, struct regcache *regcache,
+			  gdb_byte *valbuf)
+{
+  int len = TYPE_LENGTH (type);
+  gdb_byte vbuf[8];
+
+  gdb_assert (len <= 8);
+  regcache->cooked_read (BPF_R0_REGNUM, vbuf);
+  memcpy (valbuf, vbuf + 8 - len, len);
+}
+
+/* Store the function return value of type TYPE from VALBUF into REGNAME.  */
+
+static void
+bpf_store_return_value (struct type *type, struct regcache *regcache,
+			const gdb_byte *valbuf)
+{
+  int len = TYPE_LENGTH (type);
+  gdb_byte vbuf[8];
+
+  gdb_assert (len <= 8);
+  memset (vbuf, 0, sizeof (vbuf));
+  memcpy (vbuf + 8 - len, valbuf, len);
+  regcache->cooked_write (BPF_R0_REGNUM, vbuf);
+}
+
+/* Handle function's return value.  */
+
+static enum return_value_convention
+bpf_return_value (struct gdbarch *gdbarch, struct value *function,
+		  struct type *type, struct regcache *regcache,
+		  gdb_byte *readbuf, const gdb_byte *writebuf)
+{
+  int len = TYPE_LENGTH (type);
+
+  if (len > 8)
+    return RETURN_VALUE_STRUCT_CONVENTION;
+
+  if (readbuf != NULL)
+    bpf_extract_return_value (type, regcache, readbuf);
+  if (writebuf != NULL)
+    bpf_store_return_value (type, regcache, writebuf);
+
+  return RETURN_VALUE_REGISTER_CONVENTION;
+}
+
+
+/* Initialize the current architecture based on INFO.  If possible, re-use an
+   architecture from ARCHES, which is a list of architectures already created
+   during this debugging session.  */
+
+static struct gdbarch *
+bpf_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
+{
+  /* If there is already a candidate, use it.  */
+  arches = gdbarch_list_lookup_by_info (arches, &info);
+  if (arches != NULL)
+    return arches->gdbarch;
+
+  /* Allocate space for the new architecture.  */
+  struct gdbarch_tdep *tdep = XCNEW (struct gdbarch_tdep);
+  struct gdbarch *gdbarch = gdbarch_alloc (&info, tdep);
+
+  /* Information about registers, etc.  */
+  set_gdbarch_num_regs (gdbarch, BPF_NUM_REGS);
+  set_gdbarch_register_name (gdbarch, bpf_register_name);
+  set_gdbarch_register_type (gdbarch, bpf_register_type);
+
+  /* Register numbers of various important registers.  */
+  set_gdbarch_sp_regnum (gdbarch, BPF_R10_REGNUM);
+  set_gdbarch_pc_regnum (gdbarch, BPF_PC_REGNUM);
+
+  /* Map DWARF2 registers to GDB registers.  */
+  set_gdbarch_dwarf2_reg_to_regnum (gdbarch, bpf_dwarf2_reg_to_regnum);
+
+  /* Call dummy code.  */
+  set_gdbarch_call_dummy_location (gdbarch, ON_STACK);
+  set_gdbarch_dummy_id (gdbarch, bpf_dummy_id);
+  set_gdbarch_push_dummy_call (gdbarch, bpf_push_dummy_call);
+
+  /* Returning results.  */
+  set_gdbarch_return_value (gdbarch, bpf_return_value);
+
+  /* Advance PC across function entry code.  */
+  set_gdbarch_skip_prologue (gdbarch, bpf_skip_prologue);
+
+  /* Stack grows downward.  */
+  set_gdbarch_inner_than (gdbarch, core_addr_lessthan);
+
+  /* Breakpoint manipulation.  */
+  set_gdbarch_breakpoint_kind_from_pc (gdbarch, bpf_breakpoint_kind_from_pc);
+  set_gdbarch_sw_breakpoint_from_kind (gdbarch, bpf_sw_breakpoint_from_kind);
+
+  /* Frame handling.  */
+  set_gdbarch_frame_args_skip (gdbarch, 8);
+
+  /* Disassembly.  */
+  set_gdbarch_print_insn (gdbarch, bpf_gdb_print_insn);
+
+  /* Hook in ABI-specific overrides, if they have been registered.  */
+  gdbarch_init_osabi (info, gdbarch);
+
+  /* Install unwinders.  */
+  frame_unwind_append_unwinder (gdbarch, &bpf_frame_unwind);
+
+  return gdbarch;
+}
+
+void _initialize_bpf_tdep ();
+void
+_initialize_bpf_tdep (void)
+{
+  register_gdbarch_init (bfd_arch_bpf, bpf_gdbarch_init);
+
+  /* Add commands 'set/show debug bpf'.  */
+  add_setshow_zuinteger_cmd ("bpf", class_maintenance,
+			     &bpf_debug_flag,
+			     _("Set BPF debugging."),
+			     _("Show BPF debugging."),
+			     _("Enables BPF specific debugging output."),
+			     NULL,
+			     &show_bpf_debug,
+			     &setdebuglist, &showdebuglist);
+}