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Revert "feat(vex): hide RDRAND and RDSEED from CPUID with --deterministic-rng" #55

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Merged
not-matthias merged 1 commit into from
Oct 30, 2026
+9 −15
Merged

Revert "feat(vex): hide RDRAND and RDSEED from CPUID with --deterministic-rng" #55

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8 changes: 2 additions & 6 deletions VEX/priv/guest_amd64_toIR.c
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Original file line number Diff line number Diff line change
Expand Up @@ -22079,12 +22079,8 @@ Long dis_ESC_0F (
if (fAddr == &amd64g_dirtyhelper_CPUID_avx2
|| fAddr == &amd64g_dirtyhelper_CPUID_avx_and_cx16) {
Bool hasF16C = (archinfo->hwcaps & VEX_HWCAPS_AMD64_F16C) != 0;
/* A deterministic RNG still serves RDRAND/RDSEED but hides them. */
Bool advertiseRng = vex_random32 == NULL;
Bool hasRDRAND = advertiseRng
&& (archinfo->hwcaps & VEX_HWCAPS_AMD64_RDRAND) != 0;
Bool hasRDSEED = advertiseRng
&& (archinfo->hwcaps & VEX_HWCAPS_AMD64_RDSEED) != 0;
Bool hasRDRAND = (archinfo->hwcaps & VEX_HWCAPS_AMD64_RDRAND) != 0;
Bool hasRDSEED = (archinfo->hwcaps & VEX_HWCAPS_AMD64_RDSEED) != 0;
args = mkIRExprVec_4(IRExpr_GSPTR(),
mkIRExpr_HWord(hasF16C ? 1 : 0),
mkIRExpr_HWord(hasRDRAND ? 1 : 0),
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3 changes: 1 addition & 2 deletions VEX/pub/libvex.h
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Expand Up @@ -689,8 +689,7 @@ extern void LibVEX_Init (
);

/* Install before guest execution. NULL preserves host RDRAND/RDSEED.
Otherwise CPUID hides them, and they return bits from this callback,
which runs on the executing guest thread. */
The callback runs on the executing guest thread and returns 32 random bits. */
extern void LibVEX_SetRandom32 ( UInt (*random32)(void) );


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9 changes: 4 additions & 5 deletions docs/xml/manual-core.xml
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Expand Up @@ -810,11 +810,10 @@ in most cases. We group the available options by rough categories.</para>
<computeroutput>/dev/random</computeroutput> and
<computeroutput>/dev/urandom</computeroutput> (resolved
through <computeroutput>/proc/self/fd</computeroutput>, so
symlinks count). On amd64, CPUID does not report
<computeroutput>RDRAND</computeroutput> and
<computeroutput>RDSEED</computeroutput>. On hosts that support them,
programs that execute them anyway draw from the calling thread's PRNG
stream, which always reports success. On arm64,
symlinks count). On amd64, <computeroutput>RDRAND</computeroutput>
and <computeroutput>RDSEED</computeroutput> draw from the calling
thread's PRNG stream and always report success. CPUID still
reports them only when the host supports them. On arm64,
<computeroutput>RNDR</computeroutput> and
<computeroutput>RNDRRS</computeroutput> draw from the same stream and
always report success, also on hosts without FEAT_RNG.
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4 changes: 2 additions & 2 deletions none/tests/amd64/deterministic_rng.stdout.exp
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@@ -1,5 +1,5 @@
cpuid rdrand: 0
cpuid rdseed: 0
cpuid rdrand: 1
cpuid rdseed: 1
rdrand16: 555555555555d78e
rdrand16: 555555555555b519
rdrand32: 00000000131a7a6b
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