Re: [PATCH v4] elf: Properly align PT_LOAD segments [BZ #28676]

[Rongwei Wang <rongwei.wang@linux.alibaba.com>]

On 12/10/21 10:24 AM, H.J. Lu wrote:
> On Thu, Dec 9, 2021 at 5:58 PM Rongwei Wang
> <rongwei.wang@linux.alibaba.com> wrote:
>>
>>
>> Hi, lhj
>>
>> Thanks for your help! It's much better than my version.
>> And I find a small bug. you can check it again if available.
>>
>> On 12/10/21 3:29 AM, H.J. Lu wrote:
>>> On Thu, Dec 09, 2021 at 10:04:32AM -0800, H.J. Lu wrote:
>>>> On Thu, Dec 09, 2021 at 07:14:10AM -0800, H.J. Lu wrote:
>>>>
>>>> Hi,
>>>>
>>>> I updated your patch.  Please take a look.
>>>>
>>>>
>>>> H.J.
>>>> --
>>>> When PT_LOAD segment alignment > the page size, allocate enough space
>>>> to ensure that the segment can be properly aligned.
>>>>
>>>> This fixes [BZ #28676].
>>>
>>>
>>> H.J.
>>> ---
>>> Changes in v4:
>>>
>>> 1. Call unmap when the second mmap fails.
>>>
>>> When PT_LOAD segment alignment > the page size, allocate enough space
>>> to ensure that the segment can be properly aligned.
>>>
>>> This fixes [BZ #28676].
>>> ---
>>>    elf/dl-load.c         |  1 +
>>>    elf/dl-load.h         |  2 +-
>>>    elf/dl-map-segments.h | 49 +++++++++++++++++++++++++++++++++++++++----
>>>    3 files changed, 47 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/elf/dl-load.c b/elf/dl-load.c
>>> index bf8957e73c..9a23590bf4 100644
>>> --- a/elf/dl-load.c
>>> +++ b/elf/dl-load.c
>>> @@ -1150,6 +1150,7 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
>>>          c->mapend = ALIGN_UP (ph->p_vaddr + ph->p_filesz, GLRO(dl_pagesize));
>>>          c->dataend = ph->p_vaddr + ph->p_filesz;
>>>          c->allocend = ph->p_vaddr + ph->p_memsz;
>>> +       c->mapalign = ph->p_align;
>>>          c->mapoff = ALIGN_DOWN (ph->p_offset, GLRO(dl_pagesize));
>>>
>>>          /* Determine whether there is a gap between the last segment
>>> diff --git a/elf/dl-load.h b/elf/dl-load.h
>>> index e329d49a81..c121e3456c 100644
>>> --- a/elf/dl-load.h
>>> +++ b/elf/dl-load.h
>>> @@ -74,7 +74,7 @@ ELF_PREFERRED_ADDRESS_DATA;
>>>       Its details have been expanded out and converted.  */
>>>    struct loadcmd
>>>    {
>>> -  ElfW(Addr) mapstart, mapend, dataend, allocend;
>>> +  ElfW(Addr) mapstart, mapend, dataend, allocend, mapalign;
>>>      ElfW(Off) mapoff;
>>>      int prot;                             /* PROT_* bits.  */
>>>    };
>>> diff --git a/elf/dl-map-segments.h b/elf/dl-map-segments.h
>>> index f9fb110ee3..f710cf2e48 100644
>>> --- a/elf/dl-map-segments.h
>>> +++ b/elf/dl-map-segments.h
>>> @@ -18,6 +18,50 @@
>>>
>>>    #include <dl-load.h>
>>>
>>> +/* Map a segment and align it properly.  */
>>> +
>>> +static __always_inline ElfW(Addr)
>>> +_dl_map_segment (const struct loadcmd *c, ElfW(Addr) mappref,
>>> +              const size_t maplength, int fd)
>>> +{
>>> +  if (__glibc_likely (c->mapalign <= GLRO(dl_pagesize)))
>>> +    return (ElfW(Addr)) __mmap ((void *) mappref, maplength, c->prot,
>>> +                             MAP_COPY|MAP_FILE, fd, c->mapoff);
>>> +
>>> +  /* If the segment alignment > the page size, allocate enough space to
>>> +     ensure that the segment can be properly aligned.  */
>>> +  ElfW(Addr) maplen = (maplength >= c->mapalign
>>> +                    ? (maplength + c->mapalign)
>>> +                    : (2 * c->mapalign));
>>> +  ElfW(Addr) map_start = (ElfW(Addr)) __mmap ((void *) mappref, maplen,
>>> +                                           PROT_NONE,
>>> +                                           MAP_ANONYMOUS|MAP_PRIVATE,
>>> +                                           -1, 0);
>>> +  if (__glibc_unlikely ((void *) map_start == MAP_FAILED))
>>> +    return map_start;
>>> +
>>> +  ElfW(Addr) map_start_aligned = ALIGN_UP (map_start, c->mapalign);
>>> +  map_start_aligned = (ElfW(Addr)) __mmap ((void *) map_start_aligned,
>>> +                                        maplength, c->prot,
>>> +                                        MAP_COPY|MAP_FILE|MAP_FIXED,
>>> +                                        fd, c->mapoff);
>>> +  if (__glibc_unlikely ((void *) map_start == MAP_FAILED))
>> Is map_start_aligned here?
>>
>>> +    __munmap ((void *) map_start, maplen);
>>> +  else
>>> +    {
>>> +      /* Unmap the unused regions.  */
>>> +      ElfW(Addr) delta = map_start_aligned - map_start;
>>> +      if (delta)
>>> +     __munmap ((void *) map_start, delta);
>>> +      ElfW(Addr) map_end = map_start_aligned + maplength;
>>> +      delta = map_start + maplen - map_end;
>>> +      if (delta)
>>> +     __munmap ((void *) map_end, delta);
>>> +    }
>>> +
>>> +  return map_start_aligned;
>>> +}
>>> +
>>>    /* This implementation assumes (as does the corresponding implementation
>>>       of _dl_unmap_segments, in dl-unmap-segments.h) that shared objects
>>>       are always laid out with all segments contiguous (or with gaps
>>> @@ -53,10 +97,7 @@ _dl_map_segments (struct link_map *l, int fd,
>>>               - MAP_BASE_ADDR (l));
>>>
>>>          /* Remember which part of the address space this object uses.  */
>>> -      l->l_map_start = (ElfW(Addr)) __mmap ((void *) mappref, maplength,
>>> -                                            c->prot,
>>> -                                            MAP_COPY|MAP_FILE,
>>> -                                            fd, c->mapoff);
>>> +      l->l_map_start = _dl_map_segment (c, mappref, maplength, fd);
>>>          if (__glibc_unlikely ((void *) l->l_map_start == MAP_FAILED))
>>>            return DL_MAP_SEGMENTS_ERROR_MAP_SEGMENT;
>>>
>>>
>> All is OK for me.
>>
>> And I am not sure whether I need to add a testcase in one new patch,
>> because I saw you had submitted the testcase:
>> https://gitlab.com/x86-glibc/glibc/-/commit/ba63c1cb2ea1b0ae421f66756771ff4c89d4a470
> 
> No, I didn't submit it.
> 
>> I will test it again, and send out PATCH v5.
> 
> Please include my testcase in your v5 patch.
Ok

Thanks.

> 
> Thanks.
> 
> 
> --
> H.J.
>