Repost: Inaccurate function parameter location in debug info with gcc 3.x and 4.0
bibo,mao
bibo.mao@intel.com
Wed May 17 01:23:00 GMT 2006
There is one method to check whether parameter location is right. Checking
translated systemtap script, including both probed function's ip address and
function location, and compare them with assembly language of this function.
you can get this function's assembly language by "objdump -DS vmlinux" command.
Thanks
bibo,mao
Yi Feng wrote:
> 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
>
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