Re: getting vDSO image into libdwfl

[Nurdin <npremji@redhat.com>]

Roland McGrath wrote:
> The dwfl_linux_proc_find_elf callback does this as a skeletal example.
> It uses this function (see libdwfl/elf-from-memory.c):
>
>     /* Reconstruct an ELF file by reading the segments out of remote memory
>        based on the ELF file header at EHDR_VMA and the ELF program headers it
>        points to.  If not null, *LOADBASEP is filled in with the difference
>        between the addresses from which the segments were read, and the
>        addresses the file headers put them at.
>
>        The function READ_MEMORY is called to copy at least MINREAD and at most
>        MAXREAD bytes from the remote memory at target address ADDRESS into the
>        local buffer at DATA; it should return -1 for errors (with code in
>        `errno'), 0 if it failed to read at least MINREAD bytes due to EOF, or
>        the number of bytes read if >= MINREAD.  ARG is passed through.  */
>
>     Elf *
>     elf_from_remote_memory (GElf_Addr ehdr_vma,
> 			    GElf_Addr *loadbasep,
> 			    ssize_t (*read_memory) (void *arg, void *data,
> 						    GElf_Addr address,
> 						    size_t minread,
> 						    size_t maxread),
> 			    void *arg)
>
> This is an internal function not exported in the DSO or declared anywhere.
> I wrote it as the necessary and separable component (and analogous to the
> BFD function bfd_elf_bfd_from_remote_memory I added for the gdb vDSO
> support).  I tested it via dwfl_linux_proc_find_elf (-p option to
> eu-addr2line et al).  (Back then, /proc/PID/mem had not been made so
> "secure", so it even worked in eu-foo without ptrace.)  I figured the
> details of interface to that code and how it integrates with everything
> would be ironed out when a real user came along.  Now here we are.
>
> The signature of the read_memory callback is arbitrary.  
> It just seemed like the roughly appropriate general thing.
> It's easy to change.
>
> This function doesn't really fit with the libelf interfaces.  It could be
> exposed by libdwfl in some fashion I suppose.  I hadn't really figured out
> what seemed right, which is why it's as it is.  I could just change its
> name and signature slightly, and export it from libdwfl.
I just exposed this method inside libdwfl.h and I haven't needed to 
change the read_memory interface at all.