Re: [PATCH v2 1/3] perf tools: Use offset instead of dwarfnum in register table.

[Masami Hiramatsu]

On Tue, 7 Feb 2017 09:20:03 +0000
Will Deacon <will.deacon@xxxxxxx> wrote:

> On Tue, Feb 07, 2017 at 09:54:51AM +0900, Masami Hiramatsu wrote:
> > On Mon, 6 Feb 2017 13:02:29 +0000
> > Will Deacon <will.deacon@xxxxxxx> wrote:
> > 
> > > On Sat, Feb 04, 2017 at 05:03:20PM +0800, Hekuang wrote:
> > > > hi
> > > > 
> > > > 在 2017/2/3 21:00, Will Deacon 写道:
> > > > >On Fri, Feb 03, 2017 at 11:06:05AM +0000, He Kuang wrote:
> > > > >>This patch changes the 'dwarfnum' to 'offset' in register table, so
> > > > >>the index of array becomes the dwarfnum (the index of each register
> > > > >>defined by DWARF) and the "offset" member means the byte-offset of the
> > > > >>register in (user_)pt_regs. This change makes the code consistent with
> > > > >>x86.
> > > > >>
> > > > >>Acked-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> > > > >>Signed-off-by: He Kuang <hekuang@xxxxxxxxxx>
> > > > >>---
> > > > >>  tools/perf/arch/arm64/util/dwarf-regs.c | 107 ++++++++++++++++----------------
> > > > >>  1 file changed, 52 insertions(+), 55 deletions(-)
> > > > >Thanks for splitting this up. Comment below.
> > > > >
> > > > >>diff --git a/tools/perf/arch/arm64/util/dwarf-regs.c b/tools/perf/arch/arm64/util/dwarf-regs.c
> > > > >>index d49efeb..090f36b 100644
> > > > >>--- a/tools/perf/arch/arm64/util/dwarf-regs.c
> > > > >>+++ b/tools/perf/arch/arm64/util/dwarf-regs.c
> > > > >>@@ -9,72 +9,69 @@
> > > > >>   */
> > > > >>  #include <stddef.h>
> > > > >>+#include <linux/ptrace.h> /* for struct user_pt_regs */
> > > > >>  #include <dwarf-regs.h>
> > > > >>-struct pt_regs_dwarfnum {
> > > > >>+struct pt_regs_offset {
> > > > >>  	const char *name;
> > > > >>-	unsigned int dwarfnum;
> > > > >>+	int offset;
> > > > >>  };
> > > > >>-#define STR(s) #s
> > > > >>-#define REG_DWARFNUM_NAME(r, num) {.name = r, .dwarfnum = num}
> > > > >>-#define GPR_DWARFNUM_NAME(num) \
> > > > >>-	{.name = STR(%x##num), .dwarfnum = num}
> > > > >>-#define REG_DWARFNUM_END {.name = NULL, .dwarfnum = 0}
> > > > >>-
> > > > >>  /*
> > > > >>   * Reference:
> > > > >>   * http://infocenter.arm.com/help/topic/com.arm.doc.ihi0057b/IHI0057B_aadwarf64.pdf
> > > > >>   */
> > > > >>-static const struct pt_regs_dwarfnum regdwarfnum_table[] = {
> > > > >>-	GPR_DWARFNUM_NAME(0),
> > > > >>-	GPR_DWARFNUM_NAME(1),
> > > > >>-	GPR_DWARFNUM_NAME(2),
> > > > >>-	GPR_DWARFNUM_NAME(3),
> > > > >>-	GPR_DWARFNUM_NAME(4),
> > > > >>-	GPR_DWARFNUM_NAME(5),
> > > > >>-	GPR_DWARFNUM_NAME(6),
> > > > >>-	GPR_DWARFNUM_NAME(7),
> > > > >>-	GPR_DWARFNUM_NAME(8),
> > > > >>-	GPR_DWARFNUM_NAME(9),
> > > > >>-	GPR_DWARFNUM_NAME(10),
> > > > >>-	GPR_DWARFNUM_NAME(11),
> > > > >>-	GPR_DWARFNUM_NAME(12),
> > > > >>-	GPR_DWARFNUM_NAME(13),
> > > > >>-	GPR_DWARFNUM_NAME(14),
> > > > >>-	GPR_DWARFNUM_NAME(15),
> > > > >>-	GPR_DWARFNUM_NAME(16),
> > > > >>-	GPR_DWARFNUM_NAME(17),
> > > > >>-	GPR_DWARFNUM_NAME(18),
> > > > >>-	GPR_DWARFNUM_NAME(19),
> > > > >>-	GPR_DWARFNUM_NAME(20),
> > > > >>-	GPR_DWARFNUM_NAME(21),
> > > > >>-	GPR_DWARFNUM_NAME(22),
> > > > >>-	GPR_DWARFNUM_NAME(23),
> > > > >>-	GPR_DWARFNUM_NAME(24),
> > > > >>-	GPR_DWARFNUM_NAME(25),
> > > > >>-	GPR_DWARFNUM_NAME(26),
> > > > >>-	GPR_DWARFNUM_NAME(27),
> > > > >>-	GPR_DWARFNUM_NAME(28),
> > > > >>-	GPR_DWARFNUM_NAME(29),
> > > > >>-	REG_DWARFNUM_NAME("%lr", 30),
> > > > >>-	REG_DWARFNUM_NAME("%sp", 31),
> > > > >>-	REG_DWARFNUM_END,
> > > > >>-};
> > > > >>+#define REG_OFFSET_NAME(r, num) {.name = "%" #r,			\
> > > > >>+			.offset = offsetof(struct user_pt_regs, regs[num])}
> > > > >Whilst this works in practice, this is undefined behaviour for "sp", since
> > > > >you'll go off the end of the regs array.
> > > > 
> > > > It's not undefined behaviour here,
> > > > struct user_pt_regs {
> > > >         __u64           regs[31];
> > > >         __u64           sp;
> > > >         __u64           pc;
> > > >         __u64           pstate;
> > > > };
> > > > user_pt_regs->regs[31] is user_pt_regs->sp and the offset value is correct.
> > > 
> > > I think it's undefined from the C standard perspective.
> > 
> > Actually, this '%sp' is used for kprobes/uprobes dynamic events, which only 
> > depend on how regs_query_register_offset()@arch/arm64/kernel/ptrace.c is implemented.
> > And also, since perf-probe uses debuginfo, it can find out the base register.
> > 
> > So we don't need to care the C standard in this file :)
> 
> Up to the perf tool maintainers really, but I expect it will irritate
> anybody running UBSAN and it's really not difficult to fix.

Hmm, sorry, I'm not sure, what the point UBSAN user be irriteted?
And what is that related to this story?

Thank you,

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>