RE: [PATCH V6 1/3] gdb: support for eBPF

Aktemur, Tankut Baris
2020-08-03T14:55:52Z

On Monday, August 3, 2020 4:03 PM, Jose E. Marchesi wrote:
> This patch adds basic support for the eBPF target: tdep and build
> machinery. The accompanying simulator is introduced in subsequent
> patches.
>
> +/* 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)

Just my two cents. The 'struct' keyword can be removed in numerous places
throughout the file. Also, NULL can be replaced with nullptr.

> +{
> + fprintf_filtered (file, _("Debugging of BPF is %s.\n"), value);
> +}
> +
> +
>
>
> +/* BPF registers */

Might look better with dot-space-space at the end.

> +
> +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",

Is it necessary to break the line?

> + 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' */

Dot-space-space at the end of the comment.

Thanks.
-Baris

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