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Re: Failures with exelib.exp testcase (was Re: minutes 2010-08-19)
On Fri, 2011-03-04 at 12:43 +0530, K.Prasad wrote:
> On Mon, Feb 14, 2011 at 09:33:26PM +0100, Mark Wielaard wrote:
> > On Mon, 2011-02-14 at 22:19 +0530, K.Prasad wrote:
> > > On Tue, Feb 08, 2011 at 02:13:35PM +0100, Mark Wielaard wrote:
> > > > On Thu, 2011-02-03 at 10:59 -0800, Roland McGrath wrote:
> > > > > What you need to do is:
> > > > >
> > > > > Elf64_Addr opd_addr;
> > > > > Dwarf_Addr bias;
> > > > > Elf_Scn *opd = dwfl_module_address_section (mod, &sym_addr, &bias);
> > > > > if (opd == NULL) ...;
> > > > > Elf_Data *data = elf_rawdata (opd, NULL);
> > > > > if (data == NULL) ...;
> > > > > Elf_Data in, out;
> > > > > out.d_buf = &final_addr;
> > > > > in.d_buf = (char *) data->d_buf + sym_addr;
> > > > > out.d_size = in.d_size = sizeof (Elf64_Addr);
> > > > > out.d_type = in.d_type = ELF_T_ADDR;
> > > > > if (elf64_xlatetom (elf, &out, &in, e_ident[EI_DATA]) == NULL) ...;
> > > > > sym_addr = opd_addr + bias;
> > > >
>
> So I made an attempt at turning the above piece of pseudocode into a patch
> for systemtap/translate.cxx and here's what it turned out to be.
>
> Upon execution the runtime experiences "Segmentation fault" at the line
> if (elf64_xlatetom (&out, &in, elf_getident (elf, NULL)[EI_DATA]) != NULL)
> and I'm trying to understand why.
>
> Any suggestions or pointers to mistakes in creation of the patch are
> welcome.
>
> ---
> translate.cxx | 25 ++++++++++++++++++++++++-
> 1 file changed, 24 insertions(+), 1 deletion(-)
>
> Index: systemtap/translate.cxx
> ===================================================================
> --- systemtap.orig/translate.cxx
> +++ systemtap/translate.cxx
> @@ -5018,7 +5018,30 @@
> || sym.st_value >= end // beyond current module,
> || sym.st_value < base)) // before first section.
> {
> - Dwarf_Addr sym_addr = sym.st_value;
> + Dwarf_Addr sym_addr;
> +#ifdef __powerpc__
You shouldn't depend on this, do a check against the e_machine type of
the elf file header to check that is it a ppc64 one.
> +// Elf64_Addr opd_addr;
> + Dwarf_Addr bias;
> + Elf_Data *data = NULL;
> + Elf_Data in, out;
> + Elf_Scn *opd = dwfl_module_address_section (m, &sym_addr, &bias);
> + Elf* elf = (dwfl_module_getelf (m, &bias));
> +
> + if (opd != NULL)
> + {
> + data = elf_rawdata (opd, NULL);
> + if (data == NULL)
> + return DWARF_CB_ABORT;
> + }
> + out.d_buf = *userdata;
userdata is a pointer to the (unused) unwindsym_dump_context pointer
provided to us as dwfl_getmodules callback, why are you using it here?
Don't you want to store the result in the opd_addr?
> + in.d_buf = (char *) data->d_buf + sym_addr;
> + out.d_size = in.d_size = sizeof (Elf64_Addr);
> + out.d_type = in.d_type = ELF_T_ADDR;
> + if (elf64_xlatetom (&out, &in, elf_getident (elf, NULL)[EI_DATA]) != NULL)
> + sym_addr = sym.st_value + bias;
You are ignoring the error case. You are reusing the original
sym.st_value, but haven't stored the newly fetched address in it (might
be better to do that in a fresh variable anyway). Note that you are
reusing bias above for both the dwfl_module_address_section () and
dwfl_module_getelf () calls, you are using the result of the second, not
the first here.
> +#else
> + sym_addr = sym.st_value;
> +#endif /* __powerpc__ */
> Dwarf_Addr save_addr = sym_addr;
> const char *secname = NULL;