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tcg: When allocating for !splitwx, begin with PROT_NONE

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There's a change in mprotect() behaviour [1] in the latest macOS
on M1 and it's not yet clear if it's going to be fixed by Apple.

In this case, instead of changing permissions of N guard pages,
we change permissions of N rwx regions.  The same number of
syscalls are required either way.

[1] https://gist.github.com/hikalium/75ae822466ee4da13cbbe486498a191f

Reviewed-by: Luis Pires <luis.pires@eldorado.org.br>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
This commit is contained in:
Richard Henderson 2021-03-10 17:41:26 -06:00
parent 22c6a9938f
commit b7da02dad0
1 changed files with 9 additions and 10 deletions
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19
tcg/region.c
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@ -770,12 +770,15 @@ static int alloc_code_gen_buffer(size_t size, int splitwx, Error **errp)
error_free_or_abort(errp);
}
prot = PROT_READ | PROT_WRITE | PROT_EXEC;
/*
* macOS 11.2 has a bug (Apple Feedback FB8994773) in which mprotect
* rejects a permission change from RWX -> NONE when reserving the
* guard pages later. We can go the other way with the same number
* of syscalls, so always begin with PROT_NONE.
*/
prot = PROT_NONE;
flags = MAP_PRIVATE | MAP_ANONYMOUS;
#ifdef CONFIG_TCG_INTERPRETER
/* The tcg interpreter does not need execute permission. */
prot = PROT_READ | PROT_WRITE;
#elif defined(CONFIG_DARWIN)
#ifdef CONFIG_DARWIN
/* Applicable to both iOS and macOS (Apple Silicon). */
if (!splitwx) {
flags |= MAP_JIT;
@ -906,11 +909,7 @@ void tcg_region_init(size_t tb_size, int splitwx, unsigned max_cpus)
}
}
if (have_prot != 0) {
/*
* macOS 11.2 has a bug (Apple Feedback FB8994773) in which mprotect
* rejects a permission change from RWX -> NONE. Guard pages are
* nice for bug detection but are not essential; ignore any failure.
*/
/* Guard pages are nice for bug detection but are not essential. */
(void)qemu_mprotect_none(end, page_size);
}
}