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Re: [PATCH] Fix reading .dwp files without .debug_tu_index


 >I'd go with something like (untested and likely improperly formatted):
 >if (dwp_file->cus)
 >   dwp_file->version = dwp_file-cus->version;
 > else if (dwp_file->tus)
 >   dwp_file->version = dwp_file->tus->version
 > else
 >   dwp_file->version = 2;

i'm ok with that change, many thanks,
do i need to resend the patch ? (and yeah, i can double check / test it
when i get home)

regarding the dependence on the order of sections - (if i am not mistaken)
yeah, it's a separate issue - i can provide more details
when i get to my laptop.

Best regards,
Alexander Shaposhnikov

On Mon, Sep 25, 2017 at 8:14 PM, Doug Evans <dje@google.com> wrote:

> On Tue, Sep 19, 2017 at 12:58 PM, Alexander Shaposhnikov
> <alexander.v.shaposhnikov@gmail.com> wrote:
> > This patch fixes segmentation fault (due to dereferencing of a null
> pointer)
> > in dwarf2read.c in the code dwp_file->cus->version !=
> dwp_file->tus->version
> > by adding defensive checks similarly to how it's already done
> > at the lines 11208 - 11215 (in the same file dwarf2read.c).
> > The issue can be reproduced with dwp packages built by llvm-dwp utility
> > (from the current upstream) since at the moment llvm-dwp doesn't create
> > .debug_tu_index section, thus dwp_file->tus will be null.
> >
> > Test plan:
> >
> > BUILD:
> > main.cpp:
> > int f() {
> >   int x = 0;
> >   int y = 1;
> >   return x + y;
> > }
> > g++ -fPIC -gsplit-dwarf -g -O0 main.cpp -o main.exe
> > llvm-dwp main.dwo -o main.exe.dwp
> > # This step is a workaround to a separate issue (unrelated to this
> patch).
> > # At the moment llvm tools & clang put .strtab section first (its index
> is
> > 1),
> > # while gcc/gold/binutils put it at the end.
> > # Unfortunately gdb (in the code reading dwps) appears to depend on the
> > order
> > # of sections,
>
> That's odd. Depends on the order of sections how?
>
> > to workaround this (to reproduce the issue which this patch
> > # aims to address) we use objcopy to do the following trick:
> > # if one asks to remove .strtab objcopy will do that but at the same time
> > # it will create a new .shstrtab section at the end.
> > objcopy --remove-section .strtab main.exe.dwp
> > RUN:
> > gdb main.exe
> > One can observe that now there is no crash and debugging functionality
> > works as expected (setting breakpoints, printing local variable,
> exploring
> > the stack).
> >
> > gdb/ChangeLog:
> > yyyy-mm-dd  Alexander Shaposhnikov <alexander.v.shaposhnikov@gmail.com>
> >
> > * dwarf2read.c: Fix segmentation fault on reading
> >
> >   .dwp files without .debug_tu_index section.
> >
> > Patch:
> >
> > diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
> >
> > index b1914cf876..547e3f034e 100644
> > --- a/gdb/dwarf2read.c
> > +++ b/gdb/dwarf2read.c
> > @@ -11185,7 +11185,8 @@ open_and_init_dwp_file (void)
> >    dwp_file->tus = create_dwp_hash_table (dwp_file, 1);
> >
> >    /* The DWP file version is stored in the hash table.  Oh well.  */
> > -  if (dwp_file->cus->version != dwp_file->tus->version)
> > +  if (dwp_file->cus && dwp_file->tus
> > +      && dwp_file->cus->version != dwp_file->tus->version)
> >      {
> >        /* Technically speaking, we should try to limp along, but this is
> >          pretty bizarre.  We use pulongest here because that's the
> established
> > @@ -11195,7 +11196,7 @@ open_and_init_dwp_file (void)
> >              pulongest (dwp_file->cus->version),
> >              pulongest (dwp_file->tus->version), dwp_name.c_str ());
> >      }
> > -  dwp_file->version = dwp_file->cus->version;
> > +  dwp_file->version = dwp_file->cus ? dwp_file->cus->version : 0;
>
> A version of 0 doesn't seem right.
> I'd go with something like (untested and likely improperly formatted):
>
>   if (dwp_file->cus)
>     dwp_file->version = dwp_file-cus->version;
>   else if (dwp_file->tus)
>     dwp_file->version = dwp_file->tus->version
>   else
>     dwp_file->version = 2;
>
> ok with that change.
>
> >
> >    if (dwp_file->version == 2)
> >      bfd_map_over_sections (dwp_file->dbfd,
> dwarf2_locate_v2_dwp_sections,
> >
> >
> > Kind regards,
> > Alexander Shaposhnikov
>


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