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GC protection for on-stack objects
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==================================
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This document describes GC protection for on-stack objects safe against an
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irregular stack layout such as stack-smashing protection performs.
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Working on the previous stack protection strategy, following problem
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occurs.(the explanation is based on
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http://lists.sourceforge.jp/pipermail/anthy-dev/2005-August/002272.html[Anthy-dev 2273]
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by Jun Inoue, and assumes that the stack grows downward).
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Take a look in following code fragment.
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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exp = "(number->string (* 32 1024))";
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scm_gc_protect_stack(&stack_start);
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str_port = scm_make_string_port(exp);
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obj = scm_read(str_port);
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obj = SCM_EVAL(obj, SCM_INTERACTION_ENV);
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scm_gc_unprotect_stack(stack_start);
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return SCM_STRING_STR(obj);
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The previous strategy assumes that `stack_start` is certainly located at lower
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than `str_port` and `obj` to cover them on the mark-and-sweep stage. But the
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actual layout breaks the assumption as follows.
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At the breakpoint immediately before `scm_gc_protect_stack()` of the code
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compiled with gcc4 ((GCC) 4.0.1 20050617 (prerelease) (Debian 4.0.0-10)
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(IA32)), actual stack layout had been reported as follows.
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----------------------------------------------------------------
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$6 = (ScmObj *) 0xbfffe7d8
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$7 = (ScmObj *) 0xbfffe7dc
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$12 = (void *) 0xbfffe7d0
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----------------------------------------------------------------
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This result indicates the storage location order inversion:
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expected: %esp < &str_port < &stack_start
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actual: %esp < &stack_start < &str_port
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So `str_port` is not protected in the case.
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- Allocate the `stack_start` dummy variable in a separated frame
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- Ensure that a code fragment that its stack must be protected is certainly
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executed on the stack grown over the frame containing `stack_start`
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To accomplish it, a series of macros which turns a function into
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stack-protected. See following steps to know how to use it.
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1) Split the code fragment off into a function. No stack protection handling is
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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eval_str(const char *exp)
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str_port = scm_make_string_port(exp);
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obj = scm_read(str_port);
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obj = SCM_EVAL(obj, SCM_INTERACTION_ENV);
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return SCM_STRING_STR(obj);
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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2) Call the function prepared in 1) with the macro `SCM_GC_PROTECTED_CALL()` as
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follows. The macro handles all of the GC protection issues. Once the function
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returned, the stack protection is disabled again. To protect another function
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call, apply the macro to the call.
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Use `SCM_GC_PROTECTED_CALL_VOID()` instead if the calee function returns
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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const char *exp, *ret;
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exp = "(number->string (* 32 1024))";
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SCM_GC_PROTECTED_CALL(ret, const char *, eval_str, (exp));
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
added
removed
sigscheme/doc/gc-protection.txt
