Created
May 19, 2013 23:42
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gcc + asm + on-stack iovec = uninitialized memory
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// Compile at -O0 and -O1 or higher and compare the output of | |
// `strace -e pwritev a.out`. At -O0: | |
// | |
// pwritev(1, [{"Hello ", 6}, {"world!", 6}, {"\n", 1}], 3, 0) = -1 ESPIPE | |
// (Illegal seek) | |
// | |
// At -O1 or higher: | |
// | |
// pwritev(1, [{"", 0}, {"H\211l$\330L\211d$\340H\215-\367\10 \0L\215%\350\10 | |
// \0H\211\\$\320L\211l"..., 4195360}, {"", 0}], 3, 0) = -1 ESPIPE (Illegal | |
// seek) | |
// | |
// Adding "memory" to the clobber list fixes it. GCC 4.7.2 bug or not? | |
#include <sys/syscall.h> | |
#include <sys/uio.h> | |
int main(void) | |
{ | |
const struct iovec vec[] = { | |
{ .iov_base="Hello ", .iov_len=sizeof("Hello ") - 1 }, | |
{ .iov_base="world!", .iov_len=sizeof("world!") - 1 }, | |
{ .iov_base="\n", .iov_len=1 } | |
}; | |
long rval; | |
__asm__ __volatile__ ( | |
"xor %%r10, %%r10;" | |
"xor %%r8, %%r8;" | |
"syscall;" | |
: "=a" (rval) | |
: "a" (SYS_pwritev), "D" (1), "S" (vec), "d" (3) | |
: "%rcx", "%r11", "%r10", "%r8" | |
); | |
return 0; | |
} |
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Can't reproduce it on gcc 4.2.4.