add comments explaining dependence on idempotency for race-free dynam…

…ics re checkedAESNI, haveAESNI, intel_flags, and sha_method. see #7863.

wolfcrypt/src/aes.c

@@ -613,6 +613,10 @@ block cipher mechanism that uses n-bit binary string parameter key with 128-bits
         #define AESNI_ALIGN 16
     #endif
 
+    /* note that all write access to these static variables must be idempotent,
+     * as arranged by Check_CPU_support_AES(), else they will be susceptible to
+     * data races.
+     */
     static int checkedAESNI = 0;
     static int haveAESNI  = 0;
     static word32 intel_flags = 0;

wolfcrypt/src/sha256.c

@@ -408,6 +408,10 @@ static int InitSha256(wc_Sha256* sha256)
                        SHA256_SSE2, SHA256_C };
 
 #ifndef WC_C_DYNAMIC_FALLBACK
+    /* note that all write access to this static variable must be idempotent,
+     * as arranged by Sha256_SetTransform(), else it will be susceptible to
+     * data races.
+     */
     static enum sha_methods sha_method = SHA256_UNSET;
 #endif
 

wolfcrypt/src/sha512.c

@@ -489,6 +489,10 @@ static int InitSha512_256(wc_Sha512* sha512)
                        SHA512_AVX1_RORX, SHA512_AVX2_RORX, SHA512_C };
 
 #ifndef WC_C_DYNAMIC_FALLBACK
+    /* note that all write access to this static variable must be idempotent,
+     * as arranged by Sha512_SetTransform(), else it will be susceptible to
+     * data races.
+     */
     static enum sha_methods sha_method = SHA512_UNSET;
 #endif