[Bug translator/2438] Can't resolve $fd argument for sys_readv and sys_writev on ppc64

["guij at cn dot ibm dot com" <sourceware-bugzilla at sourceware dot org>]

------- Additional Comments From guij at cn dot ibm dot com  2006-04-24 09:07 -------
I met the same problem on my RHEL4U2 ppc64/2.6.16.9. i.e. sys_readv
is defined as:
  sys_readv(unsigned long fd, const struct iovec __user *vec, 
    unsigned long vlen)
but $fd cannot be resolved while both $vec and $vlen are ok.

The error message looks like:
 probe sys_readv@fs/read_write.c:599 pc=0xc0000000000b3f54
 semantic error: unresolved target-symbol expression: identifier '$fd'
   at a.stp:3:8

I did some debugging and thought the probable reason is: 
the instruction at the probe address(0xc0000000000b3f54) will modify 
the register(r3) which contains $fd, thus make related location list entry 
invalid. As a result, the effort to resolving $fd will fail even if such
DW_AT_location entry for $fd exists in .debug_loc section.

Here are the details.

I use "readelf -wi" to get:

 <1><60a219>: Abbrev Number: 73 (DW_TAG_subprogram)
     DW_AT_sibling     : <60a314>
     DW_AT_external    : 1
     DW_AT_name        : (indirect string,offset: 0x3bb14):sys_readv
     DW_AT_decl_file   : 1
     DW_AT_decl_line   : 599
     DW_AT_prototyped  : 1
     DW_AT_type        : <6026c4>
     DW_AT_low_pc      : 0xb3f44
     DW_AT_high_pc     : 0xb4004
     DW_AT_frame_base  : 0x12f94c        (location list)
 <2><60a23f>: Abbrev Number: 75 (DW_TAG_formal_parameter)
     DW_AT_name        : fd
     DW_AT_decl_file   : 1
     DW_AT_decl_line   : 598
     DW_AT_type        : <6023ef>
     DW_AT_location    : 0x12f984        (location list)
 <2><60a24e>: Abbrev Number: 75 (DW_TAG_formal_parameter)
     DW_AT_name        : vec
     DW_AT_decl_file   : 1
     DW_AT_decl_line   : 598
     DW_AT_type        : <607659>
     DW_AT_location    : 0x12f9ba        (location list)

The corresponding DW_AT_location entries in .debug_loc is 
(by eu-readelf --debug-dump=loc):
 [12f94c]  0x00000000000011a8..0x00000000000011c8 [   0] reg1
           0x00000000000011c8..0x0000000000001268 [   0] breg1 160
 [12f984]  0x00000000000011a8..0x00000000000011b8 [   0] reg3
           0x00000000000011bc..0x00000000000011d4 [   0] reg3
 [12f9ba]  0x00000000000011a8..0x00000000000011cc [   0] reg4
           0x00000000000011cc..0x00000000000011e4 [   0] reg28
           0x00000000000011e4..0x00000000000011f8 [   0] reg4
           0x0000000000001234..0x0000000000001268 [   0] reg4

The runtime base address is 0xc0000000000b2d9c, thus the valid address 
range for $fd is (0xc0000000000b3f44, 0xc0000000000b3f54). 

Unfortunately, the probe address is exactly the upper limit and thus
invalid. As a result, the elfutils interface dwarf_getlocation_addr() 
will return failure and dwflpp::translate_location() in systemtap will 
throw the semantic error.

I use "objdump -D" to get the disassembly code around sys_readv:

c0000000000b3f44 <.sys_readv>:
c0000000000b3f44:       7c 08 02 a6     mflr    r0
c0000000000b3f48:       fb 81 ff e0     std     r28,-32(r1)
c0000000000b3f4c:       fb a1 ff e8     std     r29,-24(r1)
c0000000000b3f50:       7c 9c 23 78     mr      r28,r4
c0000000000b3f54:       78 63 00 20     clrldi  r3,r3,32
c0000000000b3f58:       7c bd 2b 78     mr      r29,r5
c0000000000b3f5c:       f8 01 00 10     std     r0,16(r1)
c0000000000b3f60:       f8 21 ff 61     stdu    r1,-160(r1)
c0000000000b3f64:       38 81 00 70     addi    r4,r1,112
c0000000000b3f68:       48 00 0b 41     bl      c0000000000b4aa8 <.fget_light>
c0000000000b3f6c:       60 00 00 00     nop
c0000000000b3f70:       7f a5 eb 78     mr      r5,r29
c0000000000b3f74:       7f 84 e3 78     mr      r4,r28
c0000000000b3f78:       38 c1 00 78     addi    r6,r1,120

Here, the instruction at 0xc0000000000b3f54 modifies incoming argument 
register r3, that's why the valid address range for $fd is 
(0xc0000000000b3f44, 0xc0000000000b3f54).

In comparison, the address range for the second argument $vec is 
(0xc0000000000b3f44, 0xc0000000000b3f68), and the probe 
at 0xc0000000000b3f54 with reference to $vec succeeds.


-- 


http://sourceware.org/bugzilla/show_bug.cgi?id=2438

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