Re: [PATCH RFC 1/1] elf: align the mapping address of LOAD segments with p_align

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

On Tue, Dec 7, 2021 at 7:05 PM Rongwei Wang
<rongwei.wang@linux.alibaba.com> wrote:
>
>
>
> On 12/8/21 10:33 AM, H.J. Lu wrote:
> > On Tue, Dec 7, 2021 at 6:14 PM Rongwei Wang
> > <rongwei.wang@linux.alibaba.com> wrote:
> >>
> >> Hi hjl
> >>
> >> On 12/6/21 10:48 PM, H.J. Lu wrote:
> >>> On Fri, Dec 3, 2021 at 9:00 PM Rongwei Wang via Libc-alpha
> >>> <libc-alpha@sourceware.org> wrote:
> >>>>
> >>>> Now, ld.so always map the LOAD segments and aligned by base
> >>>> page size (e.g. 4k in x86 or 4k, 16k and 64k in arm64). And
> >>>> this patch improve the scheme here. In this patch, ld.so
> >>>> can align the mapping address of the first LOAD segment with
> >>>> p_align when p_align is greater than the current base page
> >>>> size.
> >>>
> >>> This is a bug fix.  Please open a glibc bug:
> >>>
> >>> https://sourceware.org/bugzilla/enter_bug.cgiOK
> >>
> >> And I requesting the account.
> >>>
> >>> with a testcase which should align variables to 2MB in the main
> >> By the way, I have a question about whether we need to align each LOAD
> >> segments? In our patch, we only fixed the mapping address for the first
> >> LOAD segment.
> >
> > I think the first one should be sufficient.   You can verify it with a
> > 2MB aligned variable in PIE:
> >
> > [hjl@gnu-cfl-2 tmp]$ cat x.c
> > #include <stdio.h>
> >
> > int foo  __attribute__((aligned(0x200000))) = 1;
> >
> > int
> > main ()
> > {
> >    printf ("foo: %p\n", &foo);
> > }
> > [hjl@gnu-cfl-2 tmp]$ gcc -no-pie x.c
> > [hjl@gnu-cfl-2 tmp]$ ./a.out
> > foo: 0x800000
> > [hjl@gnu-cfl-2 tmp]$ gcc x.c -fpie -pie
> > [hjl@gnu-cfl-2 tmp]$ ./a.out
> > foo: 0x55c529afe000
> > [hjl@gnu-cfl-2 tmp]$
>
> Learned it!

I opened:

https://sourceware.org/bugzilla/show_bug.cgi?id=28676

> Thanks.
> >
> >>> program and a shared library.   Please include the testcase in
> >>> your patch and mention the glibc bug in your commit message.
> >>>
> >>>> And this change makes code segments using huge pages become
> >>>> simple and available.
> >>>>
> >>>> Signed-off-by: Rongwei Wang <rongwei.wang@linux.alibaba.com>
> >>>> ---
> >>>>    elf/dl-load.c         |  1 +
> >>>>    elf/dl-map-segments.h | 54 +++++++++++++++++++++++++++++++++++++++++--
> >>>>    include/link.h        |  3 +++
> >>>>    3 files changed, 56 insertions(+), 2 deletions(-)
> >>>>
> >>>> diff --git a/elf/dl-load.c b/elf/dl-load.c
> >>>> index e39980fb19..136cfe2fa8 100644
> >>>> --- a/elf/dl-load.c
> >>>> +++ b/elf/dl-load.c
> >>>> @@ -1154,6 +1154,7 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
> >>>>             c->dataend = ph->p_vaddr + ph->p_filesz;
> >>>>             c->allocend = ph->p_vaddr + ph->p_memsz;
> >>>>             c->mapoff = ALIGN_DOWN (ph->p_offset, GLRO(dl_pagesize));
> >>>> +          l->l_load_align = ph->p_align;
> >>>>
> >>>>             /* Determine whether there is a gap between the last segment
> >>>>                and this one.  */
> >>>> diff --git a/elf/dl-map-segments.h b/elf/dl-map-segments.h
> >>>> index ac9f09ab4c..ae03236045 100644
> >>>> --- a/elf/dl-map-segments.h
> >>>> +++ b/elf/dl-map-segments.h
> >>>> @@ -18,6 +18,47 @@
> >>>>
> >>>>    #include <dl-load.h>
> >>>>
> >>>> +static __always_inline void *
> >>>> +_dl_map_segments_align (const struct loadcmd *c,
> >>>> +                   ElfW(Addr) mappref, int fd, size_t alignment,
> >>>> +                   const size_t maplength)
> >>>> +{
> >>>> +       unsigned long map_start, map_start_align, map_end;
> >>>> +       unsigned long maplen = (maplength >= alignment) ?
> >>>> +                               (maplength + alignment) : (2 * alignment);
> >>>> +
> >>>> +       /* Allocate enough space to ensure that address aligned by
> >>>> +           p_align is included. */
> >>>> +       map_start = (ElfW(Addr)) __mmap ((void *) mappref, maplen,
> >>>> +                                    PROT_NONE,
> >>>> +                                    MAP_ANONYMOUS | MAP_PRIVATE,
> >>>> +                                    -1, 0);
> >>>> +       if (__glibc_unlikely ((void *) map_start == MAP_FAILED)) {
> >>>> +               /* If mapping a aligned address failed, then ... */
> >>>> +               map_start = (ElfW(Addr)) __mmap ((void *) mappref, maplength,
> >>>> +                                    c->prot,
> >>>> +                                    MAP_COPY|MAP_FILE,
> >>>> +                                    fd, c->mapoff);
> >>>> +
> >>>> +               return (void *) map_start;
> >>>> +       }
> >>>> +       map_start_align = ALIGN_UP(map_start, alignment);
> >>>> +       map_end = map_start_align + maplength;
> >>>> +
> >>>> +       /* Remember which part of the address space this object uses.  */
> >>>> +       map_start_align = (ElfW(Addr)) __mmap ((void *) map_start_align, maplength,
> >>>> +                                    c->prot,
> >>>> +                                    MAP_COPY|MAP_FILE|MAP_FIXED,
> >>>> +                                    fd, c->mapoff);
> >>>> +       if (__glibc_unlikely ((void *) map_start_align == MAP_FAILED))
> >>>> +               return MAP_FAILED;
> >>>> +       if (map_start_align > map_start)
> >>>> +               __munmap((void *)map_start, map_start_align - map_start);
> >>>> +       __munmap((void *)map_end, map_start + maplen - map_end);
> >>>> +
> >>>> +       return (void *) map_start_align;
> >>>> +}
> >>>> +
> >>>
> >>> Please follow the glibc coding format.
> >>>
> >>>>    /* 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
> >>>> @@ -52,11 +93,20 @@ _dl_map_segments (struct link_map *l, int fd,
> >>>>                                      c->mapstart & GLRO(dl_use_load_bias))
> >>>>               - MAP_BASE_ADDR (l));
> >>>>
> >>>> -      /* Remember which part of the address space this object uses.  */
> >>>> -      l->l_map_start = (ElfW(Addr)) __mmap ((void *) mappref, maplength,
> >>>> +       /* During mapping, align the mapping address of the LOAD segments
> >>>> +          according to own p_align. This helps OS map its code segment to
> >>>> +          huge pages. */
> >>>> +       if (l->l_load_align > GLRO(dl_pagesize)) {
> >>>> +               l->l_map_start = (ElfW(Addr)) _dl_map_segments_align (c,
> >>>> +                                            mappref, fd,
> >>>> +                                            l->l_load_align, maplength);
> >>>> +       } else {
> >>>> +               /* 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);
> >>>
> >>> Please follow the glibc coding format.
> >> OK
> >>
> >> Thanks.
> >>>
> >>>> +       }
> >>>>          if (__glibc_unlikely ((void *) l->l_map_start == MAP_FAILED))
> >>>>            return DL_MAP_SEGMENTS_ERROR_MAP_SEGMENT;
> >>>>
> >>>> diff --git a/include/link.h b/include/link.h
> >>>> index aea268439c..fc6ce29fab 100644
> >>>> --- a/include/link.h
> >>>> +++ b/include/link.h
> >>>> @@ -298,6 +298,9 @@ struct link_map
> >>>>
> >>>>        /* Thread-local storage related info.  */
> >>>>
> >>>> +    /* Alignment requirement of the LOAD block.  */
> >>>> +    size_t l_load_align;
> >>>> +
> >>>>        /* Start of the initialization image.  */
> >>>>        void *l_tls_initimage;
> >>>>        /* Size of the initialization image.  */
> >>>> --
> >>>> 2.27.0
> >>>>
> >>>
> >>> Thanks.
> >>>
> >
> >
> >



-- 
H.J.