diff mbox series

[v3] elf: Properly align PT_LOAD segments [BZ #28676]

Message ID YbJFMFz5Ni7tzDMm@gmail.com
State Superseded
Headers show
Series [v3] elf: Properly align PT_LOAD segments [BZ #28676] | expand

Checks

Context Check Description
dj/TryBot-apply_patch success Patch applied to master at the time it was sent
dj/TryBot-32bit success Build for i686

Commit Message

H.J. Lu Dec. 9, 2021, 6:04 p.m. UTC 
On Thu, Dec 09, 2021 at 07:14:10AM -0800, H.J. Lu wrote:
> On Wed, Dec 8, 2021 at 9:57 PM Rongwei Wang
> <rongwei.wang@linux.alibaba.com> 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). This
> > is a bug, and had been reported:
> >
> > https://sourceware.org/bugzilla/show_bug.cgi?id=28676
> >
> > This patch mainly to fix it. 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.
> >
> > A testcase:
> > main.c:
> >
> > extern void dso_test(void);
> > int main(void)
> > {
> >     dso_test();
> >     getchar();
> >
> >     return 0;
> > }
> >
> > load.c, used to generate libload.so:
> >
> > int foo  __attribute__((aligned(0x200000))) = 1;
> > void dso_test(void)
> > {
> >     printf("dso test\n");
> >     printf("foo: %p\n", &foo);
> > }
> >
> > The steps:
> > $ gcc -O2 -fPIC -c -o load.o load.c
> > $ gcc -shared -Wl,-z,max-page-size=0x200000 -o libload.so load.o
> > $ gcc -no-pie -Wl,-z,max-page-size=0x200000 -O2 -o dso main.c libload.so -Wl,-R,.
> >
> > Before fixing:
> > $ ./dso
> > dso test
> > foo: 0xffff88ae2000
> >
> > After fixed:
> > $ ./dso
> > dso test
> > foo: 0xffff9e000000
> >
> > And this fix can help code segments use huge pages become
> > simple and available.
> 
> Please include a testcase, like
> 
> https://gitlab.com/x86-glibc/glibc/-/commits/users/hjl/pr28676/master
> 
> > Signed-off-by: Xu Yu <xuyu@linux.alibaba.com>
> > Signed-off-by: Rongwei Wang <rongwei.wang@linux.alibaba.com>
> > ---
> >  elf/dl-load.c         |  1 +
> >  elf/dl-map-segments.h | 63 +++++++++++++++++++++++++++++++++++++++----
> >  include/link.h        |  3 +++
> >  3 files changed, 62 insertions(+), 5 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;
> 
> Can you add an alignment field to
> 
> /* This structure describes one PT_LOAD command.
>    Its details have been expanded out and converted.  */
> struct loadcmd
> {
>   ElfW(Addr) mapstart, mapend, dataend, allocend;
>   ElfW(Off) mapoff;
>   int prot;                             /* PROT_* bits.  */
> };
> 
> instead?
> 

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].
---
 elf/dl-load.c         |  1 +
 elf/dl-load.h         |  2 +-
 elf/dl-map-segments.h | 51 +++++++++++++++++++++++++++++++++++++++----
 3 files changed, 49 insertions(+), 5 deletions(-)
diff mbox series

Patch

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..f147ec232f 100644
--- a/elf/dl-map-segments.h
+++ b/elf/dl-map-segments.h
@@ -18,6 +18,52 @@ 
 
 #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 (c->mapalign > GLRO(dl_pagesize))
+    {
+      /* 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);
+      ElfW(Addr) map_end = map_start_aligned + maplength;
+      map_start_aligned
+	= (ElfW(Addr)) __mmap ((void *) map_start_aligned,
+			       maplength, c->prot,
+			       MAP_COPY|MAP_FILE|MAP_FIXED,
+			       fd, c->mapoff);
+      if (__glibc_likely ((void *) map_start_aligned != MAP_FAILED))
+	{
+	  /* Unmap the unused regions.  */
+	  ElfW(Addr) delta = map_start_aligned - map_start;
+	  if (delta)
+	    __munmap ((void *) map_start, delta);
+	  delta = map_start + maplen - map_end;
+	  if (delta)
+	    __munmap ((void *) map_end, delta);
+	}
+
+      return map_start_aligned;
+    }
+
+  return (ElfW(Addr)) __mmap ((void *) mappref, maplength,
+			      c->prot, MAP_COPY|MAP_FILE,
+			      fd, c->mapoff);
+}
+
 /* 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 +99,7 @@  _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;