mcount-dynamic: Use MAP_FIXED_NOREPLACE for trampoline #1798
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By trying to allocate the extra page for a trampoline with MAP_FIXED, Uftrace could end up overlapping already mapped pages, which are discarded. Typically, if data was there with rw- protection, now the data is undefined and has rwx protection. Since Linux 4.17, the flag MAP_FIXED_NOREPLACE can be used to ensure that the fixed location is mapped atomically, preventing any clobbering. For earlier kernel, the call to mmap(2) will fall back to a random address, by not honoring the MAP_FIXED request at all. So callers should ensure that the returned address matches the requested one. Signed-off-by: Olivier Dion <odion@efficios.com>
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Hi @oldiob, thanks for the PR. We need to think a bit more about the new |
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It seems the header defines the flag as a preprocessor macro. Then we can simply check it like: #ifndef MAP_FIXED_NOREPLACE
#define MAP_FIXED_NOREPLACE MAP_FIXED
#endif |
namhyung
approved these changes
Sep 4, 2023
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By trying to allocate the extra page for a trampoline with MAP_FIXED, Uftrace could end up overlapping already mapped pages, which are discarded. Typically, if data was there with rw- protection, now the data is undefined and has rwx protection.
Since Linux 4.17, the flag MAP_FIXED_NOREPLACE can be used to ensure that the fixed location is mapped atomically, preventing any clobbering.
For earlier kernel, the call to mmap(2) will fall back to a random address, by not honoring the MAP_FIXED request at all. So callers should ensure that the returned address matches the requested one.