Re: [PATCH 1/3] elf: Introduce rtld_setup_main_map

["H.J. Lu" <hjl.tools@gmail.com>]

On Mon, Jan 3, 2022 at 9:12 AM Florian Weimer via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> This function collects most of the processing needed to initialize
> the link map for the main executable.
> ---
>  elf/rtld.c | 303 ++++++++++++++++++++++++++++-------------------------
>  1 file changed, 159 insertions(+), 144 deletions(-)
>
> diff --git a/elf/rtld.c b/elf/rtld.c
> index 24e48bf3fa..ba6e31377d 100644
> --- a/elf/rtld.c
> +++ b/elf/rtld.c
> @@ -1126,17 +1126,172 @@ rtld_chain_load (struct link_map *main_map, char *argv0)
>                      rtld_soname, pathname, errcode);
>  }
>
> +/* Called to complete the initialization of the link map for the main
> +   executable.  Returns true if there is a PT_INTERP segment.  */
> +static bool
> +rtld_setup_main_map (struct link_map *main_map)
> +{
> +  /* This have already been filled in right after _dl_new_object, or
> +     as part of _dl_map_object.  */
> +  const ElfW(Phdr) *phdr = main_map->l_phdr;
> +  ElfW(Word) phnum = main_map->l_phnum;
> +
> +  bool has_interp = false;
> +
> +  main_map->l_map_end = 0;
> +  main_map->l_text_end = 0;
> +  /* Perhaps the executable has no PT_LOAD header entries at all.  */
> +  main_map->l_map_start = ~0;
> +  /* And it was opened directly.  */
> +  ++main_map->l_direct_opencount;
> +
> +  /* Scan the program header table for the dynamic section.  */
> +  for (const ElfW(Phdr) *ph = phdr; ph < &phdr[phnum]; ++ph)
> +    switch (ph->p_type)
> +      {
> +      case PT_PHDR:
> +       /* Find out the load address.  */
> +       main_map->l_addr = (ElfW(Addr)) phdr - ph->p_vaddr;
> +       break;
> +      case PT_DYNAMIC:
> +       /* This tells us where to find the dynamic section,
> +          which tells us everything we need to do.  */
> +       main_map->l_ld = (void *) main_map->l_addr + ph->p_vaddr;
> +       main_map->l_ld_readonly = (ph->p_flags & PF_W) == 0;
> +       break;
> +      case PT_INTERP:
> +       /* This "interpreter segment" was used by the program loader to
> +          find the program interpreter, which is this program itself, the
> +          dynamic linker.  We note what name finds us, so that a future
> +          dlopen call or DT_NEEDED entry, for something that wants to link
> +          against the dynamic linker as a shared library, will know that
> +          the shared object is already loaded.  */
> +       _dl_rtld_libname.name = ((const char *) main_map->l_addr
> +                                + ph->p_vaddr);
> +       /* _dl_rtld_libname.next = NULL;        Already zero.  */
> +       GL(dl_rtld_map).l_libname = &_dl_rtld_libname;
> +
> +       /* Ordinarilly, we would get additional names for the loader from
> +          our DT_SONAME.  This can't happen if we were actually linked as
> +          a static executable (detect this case when we have no DYNAMIC).
> +          If so, assume the filename component of the interpreter path to
> +          be our SONAME, and add it to our name list.  */
> +       if (GL(dl_rtld_map).l_ld == NULL)
> +         {
> +           const char *p = NULL;
> +           const char *cp = _dl_rtld_libname.name;
> +
> +           /* Find the filename part of the path.  */
> +           while (*cp != '\0')
> +             if (*cp++ == '/')
> +               p = cp;
> +
> +           if (p != NULL)
> +             {
> +               _dl_rtld_libname2.name = p;
> +               /* _dl_rtld_libname2.next = NULL;  Already zero.  */
> +               _dl_rtld_libname.next = &_dl_rtld_libname2;
> +             }
> +         }
> +
> +       has_interp = true;
> +       break;
> +      case PT_LOAD:
> +       {
> +         ElfW(Addr) mapstart;
> +         ElfW(Addr) allocend;
> +
> +         /* Remember where the main program starts in memory.  */
> +         mapstart = (main_map->l_addr
> +                     + (ph->p_vaddr & ~(GLRO(dl_pagesize) - 1)));
> +         if (main_map->l_map_start > mapstart)
> +           main_map->l_map_start = mapstart;
> +
> +         /* Also where it ends.  */
> +         allocend = main_map->l_addr + ph->p_vaddr + ph->p_memsz;
> +         if (main_map->l_map_end < allocend)
> +           main_map->l_map_end = allocend;
> +         if ((ph->p_flags & PF_X) && allocend > main_map->l_text_end)
> +           main_map->l_text_end = allocend;
> +       }
> +       break;
> +
> +      case PT_TLS:
> +       if (ph->p_memsz > 0)
> +         {
> +           /* Note that in the case the dynamic linker we duplicate work
> +              here since we read the PT_TLS entry already in
> +              _dl_start_final.  But the result is repeatable so do not
> +              check for this special but unimportant case.  */
> +           main_map->l_tls_blocksize = ph->p_memsz;
> +           main_map->l_tls_align = ph->p_align;
> +           if (ph->p_align == 0)
> +             main_map->l_tls_firstbyte_offset = 0;
> +           else
> +             main_map->l_tls_firstbyte_offset = (ph->p_vaddr
> +                                                 & (ph->p_align - 1));
> +           main_map->l_tls_initimage_size = ph->p_filesz;
> +           main_map->l_tls_initimage = (void *) ph->p_vaddr;
> +
> +           /* This image gets the ID one.  */
> +           GL(dl_tls_max_dtv_idx) = main_map->l_tls_modid = 1;
> +         }
> +       break;
> +
> +      case PT_GNU_STACK:
> +       GL(dl_stack_flags) = ph->p_flags;
> +       break;
> +
> +      case PT_GNU_RELRO:
> +       main_map->l_relro_addr = ph->p_vaddr;
> +       main_map->l_relro_size = ph->p_memsz;
> +       break;
> +      }
> +  /* Process program headers again, but scan them backwards so
> +     that PT_NOTE can be skipped if PT_GNU_PROPERTY exits.  */
> +  for (const ElfW(Phdr) *ph = &phdr[phnum]; ph != phdr; --ph)
> +    switch (ph[-1].p_type)
> +      {
> +      case PT_NOTE:
> +       _dl_process_pt_note (main_map, -1, &ph[-1]);
> +       break;
> +      case PT_GNU_PROPERTY:
> +       _dl_process_pt_gnu_property (main_map, -1, &ph[-1]);
> +       break;
> +      }
> +
> +  /* Adjust the address of the TLS initialization image in case
> +     the executable is actually an ET_DYN object.  */
> +  if (main_map->l_tls_initimage != NULL)
> +    main_map->l_tls_initimage
> +      = (char *) main_map->l_tls_initimage + main_map->l_addr;
> +  if (! main_map->l_map_end)
> +    main_map->l_map_end = ~0;
> +  if (! main_map->l_text_end)
> +    main_map->l_text_end = ~0;
> +  if (! GL(dl_rtld_map).l_libname && GL(dl_rtld_map).l_name)
> +    {
> +      /* We were invoked directly, so the program might not have a
> +        PT_INTERP.  */
> +      _dl_rtld_libname.name = GL(dl_rtld_map).l_name;
> +      /* _dl_rtld_libname.next = NULL; Already zero.  */
> +      GL(dl_rtld_map).l_libname =  &_dl_rtld_libname;
> +    }
> +  else
> +    assert (GL(dl_rtld_map).l_libname); /* How else did we get here?  */
> +
> +  return has_interp;
> +}
> +
>  static void
>  dl_main (const ElfW(Phdr) *phdr,
>          ElfW(Word) phnum,
>          ElfW(Addr) *user_entry,
>          ElfW(auxv_t) *auxv)
>  {
> -  const ElfW(Phdr) *ph;
>    struct link_map *main_map;
>    size_t file_size;
>    char *file;
> -  bool has_interp = false;
>    unsigned int i;
>    bool prelinked = false;
>    bool rtld_is_main = false;
> @@ -1350,7 +1505,7 @@ dl_main (const ElfW(Phdr) *phdr,
>          load the program below unless it has a PT_GNU_STACK indicating
>          nonexecutable stack is ok.  */
>
> -      for (ph = phdr; ph < &phdr[phnum]; ++ph)
> +      for (const ElfW(Phdr) *ph = phdr; ph < &phdr[phnum]; ++ph)
>         if (ph->p_type == PT_GNU_STACK)
>           {
>             GL(dl_stack_flags) = ph->p_flags;
> @@ -1469,147 +1624,7 @@ dl_main (const ElfW(Phdr) *phdr,
>          information for the program.  */
>      }
>
> -  main_map->l_map_end = 0;
> -  main_map->l_text_end = 0;
> -  /* Perhaps the executable has no PT_LOAD header entries at all.  */
> -  main_map->l_map_start = ~0;
> -  /* And it was opened directly.  */
> -  ++main_map->l_direct_opencount;
> -
> -  /* Scan the program header table for the dynamic section.  */
> -  for (ph = phdr; ph < &phdr[phnum]; ++ph)
> -    switch (ph->p_type)
> -      {
> -      case PT_PHDR:
> -       /* Find out the load address.  */
> -       main_map->l_addr = (ElfW(Addr)) phdr - ph->p_vaddr;
> -       break;
> -      case PT_DYNAMIC:
> -       /* This tells us where to find the dynamic section,
> -          which tells us everything we need to do.  */
> -       main_map->l_ld = (void *) main_map->l_addr + ph->p_vaddr;
> -       main_map->l_ld_readonly = (ph->p_flags & PF_W) == 0;
> -       break;
> -      case PT_INTERP:
> -       /* This "interpreter segment" was used by the program loader to
> -          find the program interpreter, which is this program itself, the
> -          dynamic linker.  We note what name finds us, so that a future
> -          dlopen call or DT_NEEDED entry, for something that wants to link
> -          against the dynamic linker as a shared library, will know that
> -          the shared object is already loaded.  */
> -       _dl_rtld_libname.name = ((const char *) main_map->l_addr
> -                                + ph->p_vaddr);
> -       /* _dl_rtld_libname.next = NULL;        Already zero.  */
> -       GL(dl_rtld_map).l_libname = &_dl_rtld_libname;
> -
> -       /* Ordinarilly, we would get additional names for the loader from
> -          our DT_SONAME.  This can't happen if we were actually linked as
> -          a static executable (detect this case when we have no DYNAMIC).
> -          If so, assume the filename component of the interpreter path to
> -          be our SONAME, and add it to our name list.  */
> -       if (GL(dl_rtld_map).l_ld == NULL)
> -         {
> -           const char *p = NULL;
> -           const char *cp = _dl_rtld_libname.name;
> -
> -           /* Find the filename part of the path.  */
> -           while (*cp != '\0')
> -             if (*cp++ == '/')
> -               p = cp;
> -
> -           if (p != NULL)
> -             {
> -               _dl_rtld_libname2.name = p;
> -               /* _dl_rtld_libname2.next = NULL;  Already zero.  */
> -               _dl_rtld_libname.next = &_dl_rtld_libname2;
> -             }
> -         }
> -
> -       has_interp = true;
> -       break;
> -      case PT_LOAD:
> -       {
> -         ElfW(Addr) mapstart;
> -         ElfW(Addr) allocend;
> -
> -         /* Remember where the main program starts in memory.  */
> -         mapstart = (main_map->l_addr
> -                     + (ph->p_vaddr & ~(GLRO(dl_pagesize) - 1)));
> -         if (main_map->l_map_start > mapstart)
> -           main_map->l_map_start = mapstart;
> -
> -         /* Also where it ends.  */
> -         allocend = main_map->l_addr + ph->p_vaddr + ph->p_memsz;
> -         if (main_map->l_map_end < allocend)
> -           main_map->l_map_end = allocend;
> -         if ((ph->p_flags & PF_X) && allocend > main_map->l_text_end)
> -           main_map->l_text_end = allocend;
> -       }
> -       break;
> -
> -      case PT_TLS:
> -       if (ph->p_memsz > 0)
> -         {
> -           /* Note that in the case the dynamic linker we duplicate work
> -              here since we read the PT_TLS entry already in
> -              _dl_start_final.  But the result is repeatable so do not
> -              check for this special but unimportant case.  */
> -           main_map->l_tls_blocksize = ph->p_memsz;
> -           main_map->l_tls_align = ph->p_align;
> -           if (ph->p_align == 0)
> -             main_map->l_tls_firstbyte_offset = 0;
> -           else
> -             main_map->l_tls_firstbyte_offset = (ph->p_vaddr
> -                                                 & (ph->p_align - 1));
> -           main_map->l_tls_initimage_size = ph->p_filesz;
> -           main_map->l_tls_initimage = (void *) ph->p_vaddr;
> -
> -           /* This image gets the ID one.  */
> -           GL(dl_tls_max_dtv_idx) = main_map->l_tls_modid = 1;
> -         }
> -       break;
> -
> -      case PT_GNU_STACK:
> -       GL(dl_stack_flags) = ph->p_flags;
> -       break;
> -
> -      case PT_GNU_RELRO:
> -       main_map->l_relro_addr = ph->p_vaddr;
> -       main_map->l_relro_size = ph->p_memsz;
> -       break;
> -      }
> -  /* Process program headers again, but scan them backwards so
> -     that PT_NOTE can be skipped if PT_GNU_PROPERTY exits.  */
> -  for (ph = &phdr[phnum]; ph != phdr; --ph)
> -    switch (ph[-1].p_type)
> -      {
> -      case PT_NOTE:
> -       _dl_process_pt_note (main_map, -1, &ph[-1]);
> -       break;
> -      case PT_GNU_PROPERTY:
> -       _dl_process_pt_gnu_property (main_map, -1, &ph[-1]);
> -       break;
> -      }
> -
> -  /* Adjust the address of the TLS initialization image in case
> -     the executable is actually an ET_DYN object.  */
> -  if (main_map->l_tls_initimage != NULL)
> -    main_map->l_tls_initimage
> -      = (char *) main_map->l_tls_initimage + main_map->l_addr;
> -  if (! main_map->l_map_end)
> -    main_map->l_map_end = ~0;
> -  if (! main_map->l_text_end)
> -    main_map->l_text_end = ~0;
> -  if (! GL(dl_rtld_map).l_libname && GL(dl_rtld_map).l_name)
> -    {
> -      /* We were invoked directly, so the program might not have a
> -        PT_INTERP.  */
> -      _dl_rtld_libname.name = GL(dl_rtld_map).l_name;
> -      /* _dl_rtld_libname.next = NULL; Already zero.  */
> -      GL(dl_rtld_map).l_libname =  &_dl_rtld_libname;
> -    }
> -  else
> -    assert (GL(dl_rtld_map).l_libname); /* How else did we get here?  */
> +  bool has_interp = rtld_setup_main_map (main_map);
>
>    /* If the current libname is different from the SONAME, add the
>       latter as well.  */
> --
> 2.33.1
>

LGTM.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

Thanks.

-- 
H.J.