[PULL 14/14] tcg: Allow top bit of SIMD_DATA_BITS to be set in simd_desc()

[Richard Henderson]

From: Peter Maydell <peter.maydell@linaro.org>

In simd_desc() we create a SIMD descriptor from various pieces
including an arbitrary data value from the caller.  We try to
sanitize these to make sure everything will fit: the 'data' value
needs to fit in the SIMD_DATA_BITS (== 22) sized field.  However we
do that sanitizing with:
   tcg_debug_assert(data == sextract32(data, 0, SIMD_DATA_BITS));

This works for the case where the data is supposed to be considered
as a signed integer (which can then be returned via simd_data()).
However, some callers want to treat the data value as unsigned.

Specifically, for the Arm SVE operations, make_svemte_desc()
assembles a data value as a collection of fields, and it needs to use
all 22 bits.  Currently if MTE is enabled then its MTEDESC SIZEM1
field may have the most significant bit set, and then it will trip
this assertion.

Loosen the assertion so that we only check that the data value will
fit into the field in some way, either as a signed or as an unsigned
value.  This means we will fail to detect some kinds of bug in the
callers, but we won't spuriously assert for intentional use of the
data field as unsigned.

Cc: qemu-stable@nongnu.org
Fixes: db432672dc50e ("tcg: Add generic vector expanders")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2601
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-ID: <20241115172515.1229393-1-peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 tcg/tcg-op-gvec.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/tcg/tcg-op-gvec.c b/tcg/tcg-op-gvec.c
index 78ee1ced80..97e4df221a 100644
--- a/tcg/tcg-op-gvec.c
+++ b/tcg/tcg-op-gvec.c
@@ -88,7 +88,20 @@ uint32_t simd_desc(uint32_t oprsz, uint32_t maxsz, int32_t 
data)
     uint32_t desc = 0;
 
     check_size_align(oprsz, maxsz, 0);
-    tcg_debug_assert(data == sextract32(data, 0, SIMD_DATA_BITS));
+
+    /*
+     * We want to check that 'data' will fit into SIMD_DATA_BITS.
+     * However, some callers want to treat the data as a signed
+     * value (which they can later get back with simd_data())
+     * and some want to treat it as an unsigned value.
+     * So here we assert only that the data will fit into the
+     * field in at least one way. This means that some invalid
+     * values from the caller will not be detected, e.g. if the
+     * caller wants to handle the value as a signed integer but
+     * incorrectly passes us 1 << (SIMD_DATA_BITS - 1).
+     */
+    tcg_debug_assert(data == sextract32(data, 0, SIMD_DATA_BITS) ||
+                     data == extract32(data, 0, SIMD_DATA_BITS));
 
     oprsz = (oprsz / 8) - 1;
     maxsz = (maxsz / 8) - 1;
-- 
2.43.0