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Tweak malloc_stack on OpenBSD to match other implementations; explain lack of guard page #54564

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merged 2 commits into from
May 24, 2024
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Tweak malloc_stack on OpenBSD to match other implementations; explain lack of guard page #54564

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3f29772
Tweak malloc_stack on OpenBSD to match other unices
fingolfin May 23, 2024
f70f722
Explain lack of guard page on OpenBSD
fingolfin May 24, 2024
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16 changes: 14 additions & 2 deletions src/gc-stacks.c
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Original file line number Diff line number Diff line change
Expand Up @@ -32,11 +32,14 @@ static void *malloc_stack(size_t bufsz) JL_NOTSAFEPOINT
void *stk = VirtualAlloc(NULL, bufsz, MEM_RESERVE | MEM_COMMIT, PAGE_READWRITE);
if (stk == NULL)
return MAP_FAILED;

// set up a guard page to detect stack overflow
DWORD dwOldProtect;
if (!VirtualProtect(stk, jl_guard_size, PAGE_READWRITE | PAGE_GUARD, &dwOldProtect)) {
VirtualFree(stk, 0, MEM_RELEASE);
return MAP_FAILED;
}

jl_atomic_fetch_add_relaxed(&num_stack_mappings, 1);
return stk;
}
Expand All @@ -56,7 +59,14 @@ static void *malloc_stack(size_t bufsz) JL_NOTSAFEPOINT
void* stk = mmap(0, bufsz, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
if (stk == MAP_FAILED)
return MAP_FAILED;
jl_atomic_fetch_add(&num_stack_mappings, 1);

// we don't set up a guard page to detect stack overflow: on OpenBSD, any
// mmap-ed region has guard page managed by the kernel, so there is no
// need for it. Additionally, a memory region used as stack (memory
// allocated with MAP_STACK option) has strict permission, and you can't
// "create" a guard page on such memory by using `mprotect` on it

jl_atomic_fetch_add_relaxed(&num_stack_mappings, 1);
return stk;
}
# else
Expand All @@ -65,13 +75,15 @@ static void *malloc_stack(size_t bufsz) JL_NOTSAFEPOINT
void* stk = mmap(0, bufsz, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
if (stk == MAP_FAILED)
return MAP_FAILED;

#if !defined(JL_HAVE_UCONTEXT) && !defined(JL_HAVE_SIGALTSTACK)
// setup a guard page to detect stack overflow
// set up a guard page to detect stack overflow
if (mprotect(stk, jl_guard_size, PROT_NONE) == -1) {
munmap(stk, bufsz);
return MAP_FAILED;
}
#endif

jl_atomic_fetch_add_relaxed(&num_stack_mappings, 1);
return stk;
}
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