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zsh 5.2 build with --enable-stack-allocation crashes on large environments


When building a copy of zsh 5.2 for personal use, I stumbled upon this
rather unusual bug.

You can reproduce it with something like this sequence of commands in
the zsh 5.2 source directory:

./configure --enable-stack-allocation



for i ({1..6}) x=$x$x$x$x

export x

Src/zsh -f

The large environment variable causes zsh to crash on launch. Equally, a
large number of environment variables has the same effect.

Debugging shows this:

 1. Program received signal SIGSEGV, Segmentation fault.
 2. 0x000000000045cdbf in popheap () at mem.c:512
 3. 512 hl->next = NULL;
 4. (gdb) bt
 5. #0 0x000000000045cdbf in popheap () at mem.c:512
 6. #1 0x00000000004706f2 in createparamtable () at params.c:772
 7. #2 0x00000000004495ef in setupvals () at init.c:1045
 8. #3 0x0000000000449c9b in zsh_main (argc=<value optimized out>,
    argv=<value optimized out>) at init.c:1659
 9. #4 0x000003fff71d1d1d in __libc_start_main (main=0x40f3a0 <main>,
    argc=1, ubp_av=0x3ffffff8368, init=<value optimized out>,
    fini=<value optimized out>,
10. rtld_fini=<value optimized out>, stack_end=0x3ffffff8358) at
11. #5 0x000000000040f2d9 in _start ()

It's having trouble dereferencing "hl", and it turns out there's a
reason for that:

 1. Breakpoint 1, popheap () at mem.c:502
 2. (gdb) print h
 3. $1 = (struct heap *) 0x3fff7fec000
 4. (gdb) print heaps
 5. $2 = (Heap) 0x3fff7fe8000
 6. (gdb) c
 7. Continuing.
 9. Breakpoint 1, popheap () at mem.c:502
10. (gdb) print h
11. $3 = (struct heap *) 0x3fff7fe8000
12. (gdb) print heaps
13. $4 = (Heap) 0x3fff7fe8000

The above breakpoint is on the munmap in popheap. The entire "heaps"
variable is being freed at one point (because "h" is freed and they
became equal for some reason). I'm not familiar enough with the code to
figure out how this is happening though.

The bug is triggered by the --enable-stack-allocation option to
configure. Without that option, zsh works as expected.

Apologies in advance for the formatting, Thunderbird still thinks it's
in WYSIWYG mode and I'm not entirely sure how this email will end up
when it churns out the plain-text form.


Charles Daffern

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