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Re: RFH/RFC: symbol index overflow in MIPS linker stubs...
- From: Thiemo Seufer <ths at networkno dot de>
- To: David Daney <ddaney at avtrex dot com>
- Cc: binutils at sourceware dot org
- Date: Wed, 7 Jun 2006 20:27:59 +0100
- Subject: Re: RFH/RFC: symbol index overflow in MIPS linker stubs...
- References: <44871E31.email@example.com>
David Daney wrote:
> I am doing some testing of the gcj java compiler that is part of
> gcc-4.2, and have come across a problem.
> This is the ld version I am using:
> $ mipsel-linux-ld --version
> GNU ld version 2.16.91 20050817
> libgcj.so.7 contains more than 2^15 symbols in its symbol table. If a
> linker stub requires an index for a symbol that has an index greater
> than 2^15, then the index gets sign extended into a negative number.
> The result is a SIGSEGV at runtime when ld.so tries to look up the
> symbol with a negative index.
Thank you for debugging this long-standing problem.
> Here is a dissassembly that shows the problem:
> # objdump -d -z -j .MIPS.stubs /lib/libgcj.so.7
> /lib/libgcj.so.7: file format elf32-tradlittlemips
> Disassembly of section .MIPS.stubs:
> 01597f40 <.MIPS.stubs>:
> 1597f40: 8f998010 lw t9,-32752(gp)
> 1597f44: 03e07821 move t7,ra
> 1597f48: 0320f809 jalr t9
> 1597f4c: 241897cd li t8,-26675
> 1597f50: 8f998010 lw t9,-32752(gp)
> 1597f54: 03e07821 move t7,ra
> 1597f58: 0320f809 jalr t9
> 1597f5c: 241897b2 li t8,-26702
> Note that the symbol index loaded into t8 is negative.
> I think we could easily change the stubs so that they used ori (an
> unsigned immediate load), instead of li. This would work for symbol
> tables of size up to 2^16.
> FWIW: I tested this theory by hand editing the stubs to use the unsigned
> immediate load and the library now runs correctly.
That's STUB_LI16 in bfd/elfxx-mips.c:635, used at line 8020. The FIXME
immediately above the latter line is also interesting.
> A second option might be to use a two instruction sequence to load t8,
> but that would change the size of the stubs. I don't know if that would
> be portable or possible.
> Comments please.
I would favour a two instruction sequence, applications will continue to
grow. There might be some compatibility traps, but at a superficial
glance I haven't found an obvious blocker.