On 15 May 2006 22:15:44 -0400, Frank Ch. Eigler <fche@redhat.com> wrote:
Something must be very odd about your build or systemtap script, since
in general parameter access is working all right. We even have a
sys_open-related test in the pass-5 test suite, and it succeeds on a
variety of kernel and compiler versions.
If you can share more details about what you're seeing (relevant
chunks of dwarf readelf data, script, "stap -vvvv" type output) ... we
may be able to help.
Below is the relevant debug info generated by "readelf -wi vmlinux".
The kernel is 2.6.14.7 and the compiler is gcc 3.4.4. The function of
interest is sys_read.
======================================
Debug info of sys_read when compiled with -O2
<1><431c01>: Abbrev Number: 76 (DW_TAG_subprogram)
DW_AT_sibling : <431cd9>
DW_AT_external : 1
DW_AT_name : (indirect string, offset: 0x1ee17): sys_read
DW_AT_decl_file : 1
DW_AT_decl_line : 329
DW_AT_prototyped : 1
DW_AT_type : <42ae88>
DW_AT_low_pc : 0xc0157767
DW_AT_high_pc : 0xc01577dc
DW_AT_frame_base : 1 byte block: 55 (DW_OP_reg5)
<2><431c1d>: Abbrev Number: 78 (DW_TAG_formal_parameter)
DW_AT_name : fd
DW_AT_decl_file : 1
DW_AT_decl_line : 328
DW_AT_type : <42a899>
DW_AT_location : 1 byte block: 50 (DW_OP_reg0)
<2><431c2a>: Abbrev Number: 78 (DW_TAG_formal_parameter)
DW_AT_name : buf
DW_AT_decl_file : 1
DW_AT_decl_line : 328
DW_AT_type : <42addd>
DW_AT_location : 1 byte block: 50 (DW_OP_reg0)
<2><431c38>: Abbrev Number: 77 (DW_TAG_formal_parameter)
DW_AT_name : (indirect string, offset: 0xdbc1): count
DW_AT_decl_file : 1
DW_AT_decl_line : 328
DW_AT_type : <42ae7d>
DW_AT_location : 1 byte block: 50 (DW_OP_reg0)
======================================
Debug info of sys_read when compiled with -Os
<1><432a89>: Abbrev Number: 76 (DW_TAG_subprogram)
DW_AT_sibling : <432b67>
DW_AT_external : 1
DW_AT_name : (indirect string, offset: 0x1ee17): sys_read
DW_AT_decl_file : 1
DW_AT_decl_line : 329
DW_AT_prototyped : 1
DW_AT_type : <42bd17>
DW_AT_low_pc : 0xc0148d36
DW_AT_high_pc : 0xc0148d97
DW_AT_frame_base : 1 byte block: 55 (DW_OP_reg5)
<2><432aa5>: Abbrev Number: 78 (DW_TAG_formal_parameter)
DW_AT_name : fd
DW_AT_decl_file : 1
DW_AT_decl_line : 328
DW_AT_type : <42b728>
DW_AT_location : 1 byte block: 50 (DW_OP_reg0)
<2><432ab2>: Abbrev Number: 78 (DW_TAG_formal_parameter)
DW_AT_name : buf
DW_AT_decl_file : 1
DW_AT_decl_line : 328
DW_AT_type : <42bc6c>
DW_AT_location : 2 byte block: 91 c (DW_OP_fbreg: 12)
<2><432ac1>: Abbrev Number: 77 (DW_TAG_formal_parameter)
DW_AT_name : (indirect string, offset: 0xdbc1): count
DW_AT_decl_file : 1
DW_AT_decl_line : 328
DW_AT_type : <42bd0c>
DW_AT_location : 2 byte block: 91 10 (DW_OP_fbreg: 16)
============================================================
Difference in stap_xxxx.c for the two cases when reading "count" in
sys_read
void function__dwarf_tvar_get_count_0 (struct context* __restrict__ c) {
struct function__dwarf_tvar_get_count_0_locals * __restrict__ l =
& c->locals[c->nesting].function__dwarf_tvar_get_count_0;
(void) l;
#define CONTEXT c
#define THIS l
if (0) goto out;
l->__retvalue = 0;
{
{
{
- THIS->__retvalue = fetch_register (0);
+ intptr_t addr;
+ intptr_t frame_base;
+ { // DWARF expression: 0x55
+ frame_base = fetch_register (5);
+ }
+ { // DWARF expression: 0x91(16)
+ {
+ intptr_t s0;
+ s0 = frame_base + 16L;
+ addr = s0;
+ }
+ }
+ THIS->__retvalue = deref (4, addr);
}
goto out;
if (0) goto deref_fault;
deref_fault:
c->last_error = "pointer dereference fault";
goto out;
}
/* pure */
}
out:
;
#undef CONTEXT
#undef THIS
}
========================
The original systemtap script
/* rwblock.stp
This is a simple get count the number of read and write operations and
accumulate the number of bytes for each.
Will Cohen
*/
global count_sys_read
global count_sys_write
global sys_read_bytes
global sys_write_bytes
probe kernel.function("sys_read")
{
++count_sys_read;
sys_read_bytes += $count;
}
probe kernel.function("sys_write")
{
++count_sys_write;
sys_write_bytes += $count;
}
probe begin { log("starting probe") }
probe end
{
log("ending probe")
log(string(sys_read_bytes) . " bytes read with "
. string(count_sys_read) . " calls to sys_read(), avg size of read "
. string(sys_read_bytes/count_sys_read) );
log(string(sys_write_bytes) . " bytes written with "
. string(count_sys_write) . " calls to sys_write(), avg size of
write "
. string(sys_write_bytes/count_sys_write) );
}
In both cases, systemtap "succeeds" executing the script except the
numbers it reports with the -O2 kernel are bogus (because it reads
"count" from the wrong location / at the wrong time).
In my system, this is very easily reproducible. You just need to
compile the kernel using gcc 3.x with -O2 and -Os.
Please help me verify if this is just my build system's problem or it
is a common problem affecting all users. Thank you very much!
Yi Feng