[PATCH] btrace: preserve call stack on function switch
Luis Machado
lgustavo@codesourcery.com
Mon Feb 6 23:56:00 GMT 2017
On 02/01/2017 03:12 AM, Markus Metzger wrote:
> On 64-bit FC25, the _dl_runtime_resolve function uses a conditional branch to
> 'call' a particular variant optimized for that system:
>
> (gdb) disas _dl_runtime_resolve_avx_opt
> Dump of assembler code for function _dl_runtime_resolve_avx_opt:
> 0x00007ffff7deeb60 <+0>: push %rax
> 0x00007ffff7deeb61 <+1>: push %rcx
> 0x00007ffff7deeb62 <+2>: push %rdx
> 0x00007ffff7deeb63 <+3>: mov $0x1,%ecx
> 0x00007ffff7deeb68 <+8>: xgetbv
> 0x00007ffff7deeb6b <+11>: mov %eax,%r11d
> 0x00007ffff7deeb6e <+14>: pop %rdx
> 0x00007ffff7deeb6f <+15>: pop %rcx
> 0x00007ffff7deeb70 <+16>: pop %rax
> 0x00007ffff7deeb71 <+17>: and $0x4,%r11d
> 0x00007ffff7deeb75 <+21>: bnd je 0x7ffff7def4a0 <_dl_runtime_resolve_sse_vex>
> End of assembler dump.
>
> When computing the function-level trace, btrace treats this as a switch from
> _dl_runtime_resolve_avx_opt to _dl_runtime_resolve_sse_vex. We know that we
> switched functions but we can't really say in which caller/callee relationship
> those two functions are.
>
> In addition to preserving the indentaion level, also preserve the caller
> information. This is a heuristic since we don't really know. But at least in
> this case, this seems to be the right thing to do.
>
> This fixes a fail in gdb.btrace/rn-dl-bind.exp on 64-bit FC25.
>
> 2017-02-01 Markus Metzger <markus.t.metzger@intel.com>
>
> * btrace.c (ftrace_new_switch): Preserve up link and flags.
> ---
> gdb/btrace.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/gdb/btrace.c b/gdb/btrace.c
> index 6d621e4..ddf6692 100644
> --- a/gdb/btrace.c
> +++ b/gdb/btrace.c
> @@ -448,9 +448,11 @@ ftrace_new_switch (struct btrace_function *prev,
> {
> struct btrace_function *bfun;
>
> - /* This is an unexplained function switch. The call stack will likely
> - be wrong at this point. */
> + /* This is an unexplained function switch. We can't really be sure about the
> + call stack, yet the best I can think of right now is to preserve it. */
> bfun = ftrace_new_function (prev, mfun, fun);
> + bfun->up = prev->up;
> + bfun->flags = prev->flags;
>
> ftrace_debug (bfun, "new switch");
>
>
I don't know much about btrace, but the patch looks reasonable given the
explanation.
From what i understood, this adds an heuristic where previously there
was none? We just declared defeat before the patch?
More information about the Gdb-patches
mailing list