[Dwarf-discuss] Question on loclist

[Jun Koi]

Hi,

Thank you all for great replies on my last question. I am quite new to
the Dwarf spec, so have quite a few newbie questions :-)

I am working on Linux, and trying to investigate kernel at run-time.
Like everybody else, I use gcc to compile Linux kernel. I have another
question regarding loclist, and appreciate your kind helps.

In kernel 2.6.16.29, file fs/namei.c, we have the code like this:

...
static long do_rmdir(int dfd, const char __user *pathname)
{
        int error = 0;
        char * name;
        struct dentry *dentry;
        struct nameidata nd;

        name = getname(pathname);
        if(IS_ERR(name))
                return PTR_ERR(name);

        error = do_path_lookup(dfd, name, LOOKUP_PARENT, &nd);
        if (error)
                goto exit;
....

I compiled kernel with debug info option, and use dwarfdump to extract
information out. Then I got something corresponding to the above code:

...
                DW_AT_name                  do_rmdir
                DW_AT_decl_file             1 fs/namei.c
                DW_AT_decl_line             1988
                DW_AT_prototyped            yes(1)
                DW_AT_type                  <193>
                DW_AT_low_pc                0xc01617a0
                DW_AT_high_pc               0xc0161873
                DW_AT_frame_base            DW_OP_reg5
<2><41023>      DW_TAG_formal_parameter
                DW_AT_name                  dfd
                DW_AT_decl_file             1 fs/namei.c
                DW_AT_decl_line             1987
                DW_AT_type                  <49>
                DW_AT_location              <loclist with 3 entries follows>
                        [ 0]<lowpc=0x2e00><highpc=0x2e19>DW_OP_reg0
                        [ 1]<lowpc=0x2e19><highpc=0x2e3f>DW_OP_reg3
                        [ 2]<lowpc=0x2ec4><highpc=0x2ec7>DW_OP_reg3
<2><41039>      DW_TAG_formal_parameter
                DW_AT_name                  pathname
                DW_AT_decl_file             1 fs/namei.c
                DW_AT_decl_line             1987
                DW_AT_type                  <64>
                DW_AT_location              <loclist with 1 entries follows>
                        [ 0]<lowpc=0x2e00><highpc=0x2e19>DW_OP_reg2
<2><41055>      DW_TAG_variable
                DW_AT_name                  error
                DW_AT_decl_file             1 fs/namei.c
                DW_AT_decl_line             1989
                DW_AT_type                  <49>
                DW_AT_location              <loclist with 3 entries follows>
                        [ 0]<lowpc=0x2e3b><highpc=0x2e3d>DW_OP_reg0
                        [ 1]<lowpc=0x2e3f><highpc=0x2e53>DW_OP_reg3
                        [ 2]<lowpc=0x2e7a><highpc=0x2ec7>DW_OP_reg3
<2><41071>      DW_TAG_variable
                DW_AT_name                  name
                DW_AT_decl_file             1 fs/namei.c
                DW_AT_decl_line             1990
                DW_AT_type                  <82>
                DW_AT_location              <loclist with 2 entries follows>
                        [ 0]<lowpc=0x2e1b><highpc=0x2eca>DW_OP_reg6
                        [ 1]<lowpc=0x2ecf><highpc=0x2ed3>DW_OP_reg6
.....


I have few questions:

1) Why the location of variable "name" is not made with plain location
expression (like DW_OP_breg5...), but loclist?

2) Why location of "name" consists of 2 entries?The 2nd entry
"[1]<lowpc=0x2ecf><highpc=0x2ed3>DW_OP_reg6" makes more sense to me,
as the range is exactly 4bytes, and name is in fact a pointer of 4
bytes.
But how about the 1st entry? What is its meaning?

3) Most important question to my project: in the case like above, how
to determine the run-time address of "name"? (Suppose that I can have
value of all the registers - I am on x86)
I read through the Dwarf2 spec many times, but still very confused here :-(


Thank you a lot,
Jun