Re: [Tinycc-devel] mksh fails to build with bound check

[Kirill Smelkov]

+grishka

Hi Thomas,

On Wed, Jan 08, 2014 at 03:31:09PM +0800, Thomas Preud'homme wrote:
> Hi Kirill,
> 
> it seems you are with Fabrice Bellard the most familiar person with bound 
> check code in tcc. Long time ago a bug was reported in Debian about mksh 
> failing to work when compiled with tcc and we later found that it was due to 
> bound check. Since you did many improvement in the last release, I tried 
> again 
> a few days ago but unfortunately it still segfaults when compiled with tcc 
> and 
> bound checking mode on.
> 
> Would you mind to take a look? Recent version of mksh have disabled the use 
> of 
> tcc -b so I use the version 38.3 of mksh (it is the version where the problem 
> was found). Once the source downloaded, you just need to go in mksh directory 
> in the source and enter CC=tcc CFLAGS=-g sh Build.sh -r. Then try running 
> mksh 
> with the most basic command: ./mksh -c true and look at the segfault. This 
> procedure is described at [0].
> 
> [0] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=537622
> 
> Anyway, even if you can't or don't want to take a look at this bug I'd like 
> to 
> thank you for improving the bound check code in tcc. It's one of its key 
> feature that makes this project cool and that people like.
> 
> Best regards,
> 
> Thomas

Sorry for being silent for so long. I've started looking into the
problem today, but before I've ever got to mksh, I've found one new bug
wrt bcheck which is now fixed. Could you please see, does it maybe fixes
mksh? If not, I'm willing to continue digging, only it will be very
slow, as my time is very tight this days...


P.S. Grishka, I think the tests "cleanup" you've done in
d5f4df09 "tests: cleanup", where you removed test1b,test2b and test3b
tests is not right - gaining `tcc -b` being able to compile itself was
hard, and tests were there to assure it will stay that working way
without regressions.

Kirill

---- 8< ----
From: Kirill Smelkov <address@hidden>
Date: Sun, 19 Jan 2014 16:35:20 +0400
Subject: [PATCH] tccrun: Mark argv area as valid for bcheck

On my x86_64 box in i386 mode with address space randomization turned off,
I've observed the following:

    tests$ ../tcc -B.. -b -run boundtest.c 1
    Runtime error: dereferencing invalid pointer
    boundtest.c:222: at 0x808da73 main()

With diagnostic patch (like in efd9d92b "lib/bcheck: Don't assume heap
goes right after bss") and bcheck traces for __bound_new_region,
__bound_ptr_indir, etc... here is how the program run looks like:

    >>> TCC

    etext:   0x8067ed8
    edata:   0x807321d
    end:     0x807d95c
    brk:     0x807e000
    stack:  0xffffd0b4
    &errno: 0xf7dbd688
    mark_invalid  0xfff80000 -      (nil)
    mark_invalid   0x80fa000 - 0x100fa000
    new  808fdb0  808ff40  101  101  fd0  ff0
    new  808ff44  808ff48  101  101  ff0  ff0
    new  808ff49  8090049  101  101  ff0 1000
    new  808fd20  808fd29  101  101  fd0  fd0
    new  808fd2c  808fd6c  101  101  fd0  fd0
    new  808fd6d  808fda0  101  101  fd0  fd0
    E: __bound_ptr_indir4(0xffffd184, 0x4)
    Runtime error: dereferencing invalid pointer
    boundtest.c:222: at 0x808ea83 main()

So we are accessing something on stack, above stack entry for compiled
main. Investigating with gdb shows that this is argv:

    tests$ gdb ../tcc
    Reading symbols from /home/kirr/src/tools/tinycc/tcc...done.
    (gdb) set args -B.. -b -run boundtest.c 1
    (gdb) r
    Starting program: /home/kirr/src/tools/tinycc/tests/../tcc -B.. -b -run 
boundtest.c 1
    warning: Could not load shared library symbols for linux-gate.so.1.
    Do you need "set solib-search-path" or "set sysroot"?

    >>> TCC

    etext:   0x8067ed8
    edata:   0x807321d
    end:     0x807d95c
    brk:     0x807e000
    stack:  0xffffd074
    &errno: 0xf7dbd688
    mark_invalid  0xfff80000 -      (nil)
    mark_invalid   0x80fa000 - 0x100fa000
    new  808fdb0  808ff40  101  101  fd0  ff0
    new  808ff44  808ff48  101  101  ff0  ff0
    new  808ff49  8090049  101  101  ff0 1000
    new  808fd20  808fd29  101  101  fd0  fd0
    new  808fd2c  808fd6c  101  101  fd0  fd0
    new  808fd6d  808fda0  101  101  fd0  fd0
    E: __bound_ptr_indir4(0xffffd144, 0x4)

    Program received signal SIGSEGV, Segmentation fault.
    0x0808ea83 in ?? ()
    (gdb) bt
    #0  0x0808ea83 in ?? ()
    #1  0x080639b3 in tcc_run (address@hidden, address@hidden, address@hidden) 
at tccrun.c:132
    #2  0x080492b0 in main (argc=6, argv=0xffffd134) at tcc.c:346
    (gdb) f 1
    #1  0x080639b3 in tcc_run (address@hidden, address@hidden, address@hidden) 
at tccrun.c:132
    132             ret = (*prog_main)(argc, argv);
    132             ret = (*prog_main)(argc, argv);
    (gdb) p argv
    $1 = (char **) 0xffffd144

So before running compiled program, mark argv as valid region and we are
done - now the test passes.

P.S. maybe it would be better to just mark the whole vector kernel passes to
program (argv, env, auxv, etc...) as valid all at once...
---
 tccrun.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/tccrun.c b/tccrun.c
index b07ab0f..55fb3d8 100644
--- a/tccrun.c
+++ b/tccrun.c
@@ -110,13 +110,30 @@ LIBTCCAPI int tcc_run(TCCState *s1, int argc, char **argv)
     if (s1->do_bounds_check) {
         void (*bound_init)(void);
         void (*bound_exit)(void);
+        void (*bound_new_region)(void *p, unsigned long size);
+        int  (*bound_delete_region)(void *p);
+        int i;
+
         /* set error function */
         rt_bound_error_msg = tcc_get_symbol_err(s1, "__bound_error_msg");
         /* XXX: use .init section so that it also work in binary ? */
         bound_init = tcc_get_symbol_err(s1, "__bound_init");
         bound_exit = tcc_get_symbol_err(s1, "__bound_exit");
+        bound_new_region = tcc_get_symbol_err(s1, "__bound_new_region");
+        bound_delete_region = tcc_get_symbol_err(s1, "__bound_delete_region");
         bound_init();
+        /* mark argv area as valid */
+        bound_new_region(argv, argc*sizeof(argv[0]));
+        for (i=0; i<argc; ++i)
+            bound_new_region(argv[i], strlen(argv[i]));
+
         ret = (*prog_main)(argc, argv);
+
+        /* unmark argv area */
+        for (i=0; i<argc; ++i)
+            bound_delete_region(argv[i]);
+        bound_delete_region(argv);
+
         bound_exit();
     } else
 #endif
-- 
1.9.rc0.143.g6fd479e