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Detect broken RDRAND during initialization

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Some CPUs advertise RDRAND in CPUID, but return 0xFFFFFFFF
unconditionally. To avoid locking up later, test RDRAND during
initialization, and if it returns 0xFFFFFFFF, mark it as nonexistent.

Fixes #588.
This commit is contained in:
Tudor Brindus
2020-05-01 21:09:22 -04:00
parent ee90110f9b
commit 4d36b0287d
1 changed files with 33 additions and 4 deletions
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37
random_seed.c
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@@ -43,12 +43,41 @@ static void do_cpuid(int regs[], int h)
#if HAS_X86_CPUID
static int get_rdrand_seed(void);
// Valid values are -1 (haven't tested), 0 (no), and 1 (yes).
static int _has_rdrand = -1;
static int has_rdrand(void)
{
// CPUID.01H:ECX.RDRAND[bit 30] == 1
int regs[4];
do_cpuid(regs, 1);
return (regs[2] & (1 << 30)) != 0;
if (_has_rdrand == -1)
{
// CPUID.01H:ECX.RDRAND[bit 30] == 1
int regs[4];
do_cpuid(regs, 1);
if (!(regs[2] & (1 << 30)))
{
_has_rdrand = 0;
} else
{
// Some CPUs advertise RDRAND in CPUID, but return 0xFFFFFFFF
// unconditionally. To avoid locking up later, test RDRAND here. If over
// 10 trials RDRAND has returned the same value, declare it broken.
_has_rdrand = 0;
int prev = get_rdrand_seed();
for (int i = 0; i < 10; i++) {
int temp = get_rdrand_seed();
if (temp != prev) {
_has_rdrand = 1;
break;
}
prev = temp;
}
}
}
return _has_rdrand;
}
#endif