------- Comment #11 from mikulas at artax dot karlin dot mff dot cuni dot cz
2009-07-31 01:00 -------
So I did this experiment whether the stack is aligned in current Linux
binaries.
I applied this patch for gcc, so that it crashes on function entry if the
function has stack not aligned on 16 bytes.
diff -urp gcc-4.4.1/gcc/varasm.c gcc-4.4.1-test-align/gcc/varasm.c
--- gcc-4.4.1/gcc/varasm.c 2009-03-17 21:18:21.000000000 +0100
+++ gcc-4.4.1-test-align/gcc/varasm.c 2009-07-25 16:18:11.000000000 +0200
@@ -1760,6 +1760,8 @@ assemble_start_function (tree decl, cons
/* Standard thing is just output label for the function. */
ASM_OUTPUT_LABEL (asm_out_file, fnname);
#endif /* ASM_DECLARE_FUNCTION_NAME */
+ if (!crtl->stack_realign_needed)
+ fputs("\tsubl\t$12, %esp\n\ttestl\t$15,
%esp\n\tjz\t99999f\n\tud2a\n99999:\taddl\t$12, %esp\n", asm_out_file);
}
/* Output assembler code associated with defining the size of the
--- and the results are terrifying:
Gcc didn't even bootstrap itself. It failed because it calls glibc function
obstack_init and it calls back to xmalloc - with misaligned stack. So I
compiled gcc without bootstrap and tried to compile glibc-2.7 with it. Glibc
compiles its integer-only code with -mpreferred-stack-boundary=2, so I changed
it to -mpreferred-stack-boundary=4.
Glibc didn't finish its build either (failed when running some self-compiled
scripts), but it at least produced libc.so.
So I tried to preload this libc.so with stack-alignment-checking to various
Linux binaries (with LD_PRELOAD) and see what happens.
Out of 95 binaries in /bin/, only 23 succeeded! The remaining crashed because
of glibc was called with unaligned stack. (the distribution is up-to-date
Debian Lenny).
The non-crashing binaries are:
bzip2recover, cpio, dmesg, fgconsole, fuser, kill, loadkeys, lsmod, lvnet,
mktemp, more (displays help only, crashes when attempting to display any file),
mount, mountpoint, mt, mt-gnu, nbd-server, pidof, ping, ping6, run-parts, sed,
su, tailf, umount
So anyone, who is saying that the stack is aligned to 16 bytes has his mind
disconnected from reality. It isn't.
I find it very unreasonable that GCC developers try to declare their own ABI
with aligned stack --- and that conflicts with what is being used by the
majority of Linux applications. GCC developers are trying to say that 3/4 of
programs in /bin/ are wrong because they don't align the stack.
I think you should really align the stack in the functions that do SSE math and
don't rely on the fact that the stack is already aligned. It is definitelly
easier to use the code for stack reallign than declaring that majority of Linux
binaries are BAD and need to be recompiled.
If some scientists needed extreme performance and can't take the penalty of
realigning the stack, you can add an option -massume-aligned-stack form them
and it is the responsibility of a given scientist that the code compiled with
this option is never called back from libc or anything else else. But don't
assume stack alignment for general code. It just isn't true.
--
mikulas at artax dot karlin dot mff dot cuni dot cz changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|RESOLVED |UNCONFIRMED
Resolution|DUPLICATE |
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40838
