fix overread in Intra4Preds_NEON

Extend VP8EncIterator::i4_boundary_ by 3 bytes to avoid Intra4Preds_NEON
reading deeper into the struct (likely padding) when top is positioned
at offset 29. This data is memset with MSan to prevent a warning due to
its incorrect modeling of tbl instructions.

Prior to:
  169dfbf disable Intra4Preds_NEON
there was a mismatch in the preprocessor checks for enabling the
function in NEON and removing the C version; NEON used `BPS == 32` while
the C code was removed unconditionally when building for aarch64. This
patch also normalizes those checks to look for `BPS == 32` and `BPS !=
32` as appropriate.

Bug: b:366668849,webp:372109644
Change-Id: Ic9e6ad4b2d844cb446decd63aec0b2676a89c8d0

src/dsp/enc.c

@@ -359,8 +359,7 @@ static void Intra16Preds_C(uint8_t* WEBP_RESTRICT dst,
 //------------------------------------------------------------------------------
 // luma 4x4 prediction
 
-// TODO: b/366668849 - Restore this condition after Intra4Preds_NEON is fixed.
-#if 1  // !WEBP_NEON_OMIT_C_CODE || !WEBP_AARCH64
+#if !WEBP_NEON_OMIT_C_CODE || !WEBP_AARCH64 || BPS != 32
 
 #define DST(x, y) dst[(x) + (y) * BPS]
 #define AVG3(a, b, c) ((uint8_t)(((a) + 2 * (b) + (c) + 2) >> 2))
@@ -551,7 +550,7 @@ static void Intra4Preds_C(uint8_t* WEBP_RESTRICT dst,
   HU4(I4HU4 + dst, top);
 }
 
-#endif  // !WEBP_NEON_OMIT_C_CODE || !WEBP_AARCH64
+#endif  // !WEBP_NEON_OMIT_C_CODE || !WEBP_AARCH64 || BPS != 32
 
 //------------------------------------------------------------------------------
 // Metric
@@ -798,9 +797,9 @@ WEBP_DSP_INIT_FUNC(VP8EncDspInit) {
   VP8EncQuantizeBlockWHT = QuantizeBlock_C;
 #endif
 
-  // TODO: b/366668849 - Move this into the #if after Intra4Preds_NEON is
-  // fixed.
+#if !WEBP_NEON_OMIT_C_CODE || !WEBP_AARCH64 || BPS != 32
   VP8EncPredLuma4 = Intra4Preds_C;
+#endif
 #if !WEBP_NEON_OMIT_C_CODE || !WEBP_AARCH64
   VP8EncPredLuma16 = Intra16Preds_C;
 #endif

src/dsp/enc_neon.c

@@ -927,8 +927,7 @@ static int Quantize2Blocks_NEON(int16_t in[32], int16_t out[32],
 
 #if WEBP_AARCH64
 
-// TODO: b/366668849 - enable Intra4Preds_NEON after fixing overread.
-#if 0
+#if BPS == 32
 #define DC4_VE4_HE4_TM4_NEON(dst, tbl, res, lane)                              \
   do {                                                                         \
     uint8x16_t r;                                                              \
@@ -1040,7 +1039,7 @@ static void Intra4Preds_NEON(uint8_t* WEBP_RESTRICT dst,
   vst1_u8(dst + I4HD4 + BPS * 2, vget_low_u8(result1));
   vst1_u8(dst + I4HD4 + BPS * 3, vget_high_u8(result1));
 }
-#endif  // 0
+#endif  // BPS == 32
 
 static WEBP_INLINE void Fill_NEON(uint8_t* dst, const uint8_t value) {
   uint8x16_t a = vdupq_n_u8(value);
@@ -1212,8 +1211,7 @@ WEBP_TSAN_IGNORE_FUNCTION void VP8EncDspInitNEON(void) {
   VP8SSE4x4 = SSE4x4_NEON;
 
 #if WEBP_AARCH64
-  // TODO: b/366668849 - enable Intra4Preds_NEON after fixing overread.
-#if 0  // BPS == 32
+#if BPS == 32
   VP8EncPredLuma4 = Intra4Preds_NEON;
 #endif
   VP8EncPredLuma16 = Intra16Preds_NEON;

src/enc/iterator_enc.c

@@ -13,6 +13,7 @@
 
 #include <string.h>
 
+#include "src/dsp/cpu.h"
 #include "src/enc/vp8i_enc.h"
 
 //------------------------------------------------------------------------------
@@ -425,6 +426,15 @@ void VP8IteratorStartI4(VP8EncIterator* const it) {
       it->i4_boundary_[17 + i] = it->i4_boundary_[17 + 15];
     }
   }
+#if WEBP_AARCH64 && BPS == 32 && defined(WEBP_MSAN)
+  // Intra4Preds_NEON() reads 3 uninitialized bytes from i4_boundary_ when top
+  // is positioned at offset 29 (VP8TopLeftI4[3]). The values are not used
+  // meaningfully, but due to limitations in MemorySanitizer related to
+  // modeling of tbl instructions, a warning will be issued. This can be
+  // removed if MSan is updated to support the instructions. See
+  // https://issues.webmproject.org/372109644.
+  memset(it->i4_boundary_ + sizeof(it->i4_boundary_) - 3, 0xaa, 3);
+#endif
   VP8IteratorNzToBytes(it);  // import the non-zero context
 }
 

src/enc/vp8i_enc.h

@@ -16,6 +16,7 @@
 
 #include <string.h>     // for memcpy()
 #include "src/dec/common_dec.h"
+#include "src/dsp/cpu.h"
 #include "src/dsp/dsp.h"
 #include "src/utils/bit_writer_utils.h"
 #include "src/utils/thread_utils.h"
@@ -233,7 +234,11 @@ typedef struct {
   VP8BitWriter* bw_;               // current bit-writer
   uint8_t*      preds_;            // intra mode predictors (4x4 blocks)
   uint32_t*     nz_;               // non-zero pattern
+#if WEBP_AARCH64 && BPS == 32
+  uint8_t       i4_boundary_[40];  // 32+8 boundary samples needed by intra4x4
+#else
   uint8_t       i4_boundary_[37];  // 32+5 boundary samples needed by intra4x4
+#endif
   uint8_t*      i4_top_;           // pointer to the current top boundary sample
   int           i4_;               // current intra4x4 mode being tested
   int           top_nz_[9];        // top-non-zero context.