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accel/tcg: Avoid reading too much in load_atom_{2,4}
When load_atom_extract_al16_or_al8 is inexpensive, we want to use it early, in order to avoid the overhead of required_atomicity. However, we must not read past the end of the page. If there are more than 8 bytes remaining, then both the "aligned 16" and "aligned 8" paths align down so that the read has at least 16 bytes remaining on the page. Reviewed-by: Peter Maydell <peter.maydell@linaro.org> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
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Original file line number | Diff line number | Diff line change |
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#include <sys/mman.h> | ||
#include <sys/shm.h> | ||
#include <unistd.h> | ||
#include <stdio.h> | ||
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int main() | ||
{ | ||
int psize = getpagesize(); | ||
int id; | ||
void *p; | ||
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/* | ||
* We need a shared mapping to enter CF_PARALLEL mode. | ||
* The easiest way to get that is shmat. | ||
*/ | ||
id = shmget(IPC_PRIVATE, 2 * psize, IPC_CREAT | 0600); | ||
if (id < 0) { | ||
perror("shmget"); | ||
return 2; | ||
} | ||
p = shmat(id, NULL, 0); | ||
if (p == MAP_FAILED) { | ||
perror("shmat"); | ||
return 2; | ||
} | ||
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/* Protect the second page. */ | ||
if (mprotect(p + psize, psize, PROT_NONE) < 0) { | ||
perror("mprotect"); | ||
return 2; | ||
} | ||
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/* | ||
* Load 4 bytes, 6 bytes from the end of the page. | ||
* On success this will load 0 from the newly allocated shm. | ||
*/ | ||
return *(int *)(p + psize - 6); | ||
} |