Commit e65f7ae7 authored by Masami Hiramatsu's avatar Masami Hiramatsu Committed by Steven Rostedt (VMware)
Browse files

tracing/probe: Support user-space dereference 

Support user-space dereference syntax for probe event arguments
to dereference the data-structure or array in user-space.

The syntax is just adding 'u' before an offset value.

 +|-u<OFFSET>(<FETCHARG>)

e.g. +u8(%ax), +u0(+0(%si))

For example, if you probe do_sched_setscheduler(pid, policy,
param) and record param->sched_priority, you can add new
probe as below;

 p do_sched_setscheduler priority=+u0($arg3)

Note that kprobe event provides this and it doesn't change the
dereference method automatically because we do not know whether
the given address is in userspace or kernel on some archs.

So as same as "ustring", this is an option for user, who has to
carefully choose the dereference method.

Link: http://lkml.kernel.org/r/155789872187.26965.4468456816590888687.stgit@devnote2



Acked-by: default avatarIngo Molnar <mingo@kernel.org>
Signed-off-by: default avatarMasami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
parent 88903c46

Documentation/trace/kprobetrace.rst

+22 −5
Original line number Diff line number Diff line
@@ -51,7 +51,7 @@ Synopsis of kprobe_events 
  $argN		: Fetch the Nth function argument. (N >= 1) (\*1)

  $retval	: Fetch return value.(\*2)

  $comm		: Fetch current task comm.

  +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(\*3)

  +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*3)(\*4)

  NAME=FETCHARG : Set NAME as the argument name of FETCHARG.

  FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types

		  (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types
@@ -61,6 +61,7 @@ Synopsis of kprobe_events 
  (\*1) only for the probe on function entry (offs == 0).

  (\*2) only for return probe.

  (\*3) this is useful for fetching a field of data structures.

  (\*4) "u" means user-space dereference. See :ref:`user_mem_access`.



Types

-----
@@ -79,10 +80,7 @@ wrong, but '+8($stack):x8[8]' is OK.) 
String type is a special type, which fetches a "null-terminated" string from

kernel space. This means it will fail and store NULL if the string container

has been paged out. "ustring" type is an alternative of string for user-space.

Note that kprobe-event provides string/ustring types, but doesn't change it

automatically. So user has to decide if the targe string in kernel or in user

space carefully. On some arch, if you choose wrong one, it always fails to

record string data.

See :ref:`user_mem_access` for more info..

The string array type is a bit different from other types. For other base

types, <base-type>[1] is equal to <base-type> (e.g. +0(%di):x32[1] is same

as +0(%di):x32.) But string[1] is not equal to string. The string type itself
@@ -97,6 +95,25 @@ Symbol type('symbol') is an alias of u32 or u64 type (depends on BITS_PER_LONG) 
which shows given pointer in "symbol+offset" style.

For $comm, the default type is "string"; any other type is invalid.



.. _user_mem_access:

User Memory Access

------------------

Kprobe events supports user-space memory access. For that purpose, you can use

either user-space dereference syntax or 'ustring' type.



The user-space dereference syntax allows you to access a field of a data

structure in user-space. This is done by adding the "u" prefix to the

dereference syntax. For example, +u4(%si) means it will read memory from the

address in the register %si offset by 4, and the memory is expected to be in

user-space. You can use this for strings too, e.g. +u0(%si):string will read

a string from the address in the register %si that is expected to be in user-

space. 'ustring' is a shortcut way of performing the same task. That is,

+0(%si):ustring is equivalent to +u0(%si):string.



Note that kprobe-event provides the user-memory access syntax but it doesn't

use it transparently. This means if you use normal dereference or string type

for user memory, it might fail, and may always fail on some archs. The user

has to carefully check if the target data is in kernel or user space.



Per-Probe Event Filtering

-------------------------

Documentation/trace/uprobetracer.rst

+6 −4
Original line number Diff line number Diff line
@@ -42,16 +42,18 @@ Synopsis of uprobe_tracer 
   @+OFFSET	: Fetch memory at OFFSET (OFFSET from same file as PATH)

   $stackN	: Fetch Nth entry of stack (N >= 0)

   $stack	: Fetch stack address.

   $retval	: Fetch return value.(*)

   $retval	: Fetch return value.(\*1)

   $comm	: Fetch current task comm.

   +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**)

   +|-[u]OFFS(FETCHARG) : Fetch memory at FETCHARG +|- OFFS address.(\*2)(\*3)

   NAME=FETCHARG     : Set NAME as the argument name of FETCHARG.

   FETCHARG:TYPE     : Set TYPE as the type of FETCHARG. Currently, basic types

		       (u8/u16/u32/u64/s8/s16/s32/s64), hexadecimal types

		       (x8/x16/x32/x64), "string" and bitfield are supported.



  (*) only for return probe.

  (**) this is useful for fetching a field of data structures.

  (\*1) only for return probe.

  (\*2) this is useful for fetching a field of data structures.

  (\*3) Unlike kprobe event, "u" prefix will just be ignored, becuse uprobe

        events can access only user-space memory.



Types

-----

kernel/trace/trace.c

+3 −2
Original line number Diff line number Diff line
@@ -4842,10 +4842,11 @@ static const char readme_msg[] = 
	"\t     args: <name>=fetcharg[:type]\n"

	"\t fetcharg: %<register>, @<address>, @<symbol>[+|-<offset>],\n"

#ifdef CONFIG_HAVE_FUNCTION_ARG_ACCESS_API

	"\t           $stack<index>, $stack, $retval, $comm, $arg<N>\n"

	"\t           $stack<index>, $stack, $retval, $comm, $arg<N>,\n"

#else

	"\t           $stack<index>, $stack, $retval, $comm\n"

	"\t           $stack<index>, $stack, $retval, $comm,\n"

#endif

	"\t           +|-[u]<offset>(<fetcharg>)\n"

	"\t     type: s8/16/32/64, u8/16/32/64, x8/16/32/64, string, symbol,\n"

	"\t           b<bit-width>@<bit-offset>/<container-size>, ustring,\n"

	"\t           <type>\\[<array-size>\\]\n"

kernel/trace/trace_kprobe.c

+6 −0
Original line number Diff line number Diff line
@@ -952,6 +952,12 @@ probe_mem_read(void *dest, void *src, size_t size) 
	return probe_kernel_read(dest, src, size);

}



static nokprobe_inline int

probe_mem_read_user(void *dest, void *src, size_t size)

{

	return probe_user_read(dest, src, size);

}



/* Note that we don't verify it, since the code does not come from user space */

static int

process_fetch_insn(struct fetch_insn *code, struct pt_regs *regs, void *dest,

kernel/trace/trace_probe.c

+18 −7
Original line number Diff line number Diff line
@@ -324,6 +324,7 @@ parse_probe_arg(char *arg, const struct fetch_type *type, 
{

	struct fetch_insn *code = *pcode;

	unsigned long param;

	int deref = FETCH_OP_DEREF;

	long offset = 0;

	char *tmp;

	int ret = 0;
@@ -396,9 +397,14 @@ parse_probe_arg(char *arg, const struct fetch_type *type, 
		break;



	case '+':	/* deref memory */

		arg++;	/* Skip '+', because kstrtol() rejects it. */

		/* fall through */

	case '-':

		if (arg[1] == 'u') {

			deref = FETCH_OP_UDEREF;

			arg[1] = arg[0];

			arg++;

		}

		if (arg[0] == '+')

			arg++;	/* Skip '+', because kstrtol() rejects it. */

		tmp = strchr(arg, '(');

		if (!tmp) {

			trace_probe_log_err(offs, DEREF_NEED_BRACE);
@@ -434,7 +440,7 @@ parse_probe_arg(char *arg, const struct fetch_type *type, 
			}

			*pcode = code;



			code->op = FETCH_OP_DEREF;

			code->op = deref;

			code->offset = offset;

		}

		break;
@@ -573,14 +579,15 @@ static int traceprobe_parse_probe_arg_body(char *arg, ssize_t *size, 
	/* Store operation */

	if (!strcmp(parg->type->name, "string") ||

	    !strcmp(parg->type->name, "ustring")) {

		if (code->op != FETCH_OP_DEREF && code->op != FETCH_OP_IMM &&

		    code->op != FETCH_OP_COMM) {

		if (code->op != FETCH_OP_DEREF && code->op != FETCH_OP_UDEREF &&

		    code->op != FETCH_OP_IMM && code->op != FETCH_OP_COMM) {

			trace_probe_log_err(offset + (t ? (t - arg) : 0),

					    BAD_STRING);

			ret = -EINVAL;

			goto fail;

		}

		if (code->op != FETCH_OP_DEREF || parg->count) {

		if ((code->op == FETCH_OP_IMM || code->op == FETCH_OP_COMM) ||

		     parg->count) {

			/*

			 * IMM and COMM is pointing actual address, those must

			 * be kept, and if parg->count != 0, this is an array
@@ -594,7 +601,8 @@ static int traceprobe_parse_probe_arg_body(char *arg, ssize_t *size, 
			}

		}

		/* If op == DEREF, replace it with STRING */

		if (!strcmp(parg->type->name, "ustring"))

		if (!strcmp(parg->type->name, "ustring") ||

		    code->op == FETCH_OP_UDEREF)

			code->op = FETCH_OP_ST_USTRING;

		else

			code->op = FETCH_OP_ST_STRING;
@@ -603,6 +611,9 @@ static int traceprobe_parse_probe_arg_body(char *arg, ssize_t *size, 
	} else if (code->op == FETCH_OP_DEREF) {

		code->op = FETCH_OP_ST_MEM;

		code->size = parg->type->size;

	} else if (code->op == FETCH_OP_UDEREF) {

		code->op = FETCH_OP_ST_UMEM;

		code->size = parg->type->size;

	} else {

		code++;

		if (code->op != FETCH_OP_NOP) {

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