[klibc] shared klibc broken on x86_64 with binutils >= 2.17.50.0.2

[Sergey Vlasov]

Hello!

I have encountered a problem when building shared klibc with recent
binutils versions - apparently the breakage happened since binutils
2.17.50.0.2 due to the following change:

http://sources.redhat.com/ml/binutils/2006-05/msg00279.html
http://sourceware.org/ml/binutils/2006-05/msg00466.html

--- a/binutils/bfd/elf64-x86-64.c
+++ b/binutils/bfd/elf64-x86-64.c
@@ -3630,7 +3630,8 @@ static const struct bfd_elf_special_section
 #define TARGET_LITTLE_NAME		    "elf64-x86-64"
 #define ELF_ARCH			    bfd_arch_i386
 #define ELF_MACHINE_CODE		    EM_X86_64
-#define ELF_MAXPAGESIZE			    0x100000
+#define ELF_MAXPAGESIZE			    0x200000
+#define ELF_MINPAGESIZE			    0x1000
 
 #define elf_backend_can_gc_sections	    1
 #define elf_backend_can_refcount	    1

When building klibc with new binutils, I get:

$ readelf -l usr/klibc/libc.so                                       

Elf file type is EXEC (Executable file)
Entry point 0x200200
There are 3 program headers, starting at offset 64

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  LOAD           0x0000000000000000 0x0000000000200000 0x0000000000200000
                 0x0000000000011a5e 0x0000000000011a5e  R E    200000
  LOAD           0x0000000000011a60 0x0000000000411a60 0x0000000000411a60
                 0x0000000000000100 0x0000000000004288  RW     200000
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000  RWE    8

 Section to Segment mapping:
  Segment Sections...
   00     .text .rodata 
   01     .data .bss 
   02     

The .text and .rodata sections are placed in a segment starting at
0x200000, but .data and .bss are placed starting at 0x411a60.  But
binaries are linked starting at 0x400000 - therefore the executable
code falls between the klibc.so segments.  This even seems to work on
mainline kernels (at least on 2.6.18 - although it seems to work just
because programs are small enough), but breaks on kernels with the
exec-shield patch (http://people.redhat.com/mingo/exec-shield/) - all
programs linked with shared klibc segfault immediately on such
kernels, because the executable code segment is not mapped into
virtual memory at all.

Older binutils versions with ELF_MAXPAGESIZE = 0x100000 generated
working klibc.so:

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  LOAD           0x0000000000000000 0x0000000000200000 0x0000000000200000
                 0x0000000000011a5e 0x0000000000011a5e  R E    100000
  LOAD           0x0000000000011a60 0x0000000000311a60 0x0000000000311a60
                 0x0000000000000100 0x0000000000004288  RW     100000
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000  RWE    8

Here the data segment was placed at 0x311a60, leaving enough space for
the main executable code at 0x400000.

Seems that the increased maximum page size is just for better
performance, and the value can be decreased without losing
compatibility.  New ld versions have an option for this:

  -z max-page-size=SIZE Set maximum page size to SIZE

Adding "-z max-page-size=0x100000" to KLIBCSHAREDFLAGS fixes klibc.so
build on new binutils - the resulting binaries run fine even on
kernels with the exec-shield patch.  Probably this option can even be
added unconditionally - GNU ld silently ignores all "-z KEYWORD"
options which it does not understand, therefore the added options
should not break linking with old ld versions (although I did not
actually test this yet).

Maybe somebody has some other solution to this problem?

-- 
Sergey Vlasov
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