fix rawbigints OOB issues (#55917)

Fixes issues introduced in #50691 and found in #55906:
* use `@inbounds` and `@boundscheck` macros in rawbigints, for catching
OOB with `--check-bounds=yes`
* fix OOB in `truncate`

base/rawbigints.jl

@@ -21,14 +21,21 @@ reversed_index(n::Int, i::Int) = n - i - 1
 reversed_index(x, i::Int, v::Val) = reversed_index(elem_count(x, v), i)::Int
 split_bit_index(x::RawBigInt, i::Int) = divrem(i, word_length(x), RoundToZero)
 
+function get_elem_words_raw(x::RawBigInt{T}, i::Int) where {T}
+    @boundscheck if (i < 0) || (elem_count(x, Val(:words)) ≤ i)
+        throw(BoundsError(x, i))
+    end
+    d = x.d
+    j = i + 1
+    (GC.@preserve d unsafe_load(Ptr{T}(pointer(d)), j))::T
+end
+
 """
 `i` is the zero-based index of the wanted word in `x`, starting from
 the less significant words.
 """
-function get_elem(x::RawBigInt{T}, i::Int, ::Val{:words}, ::Val{:ascending}) where {T}
-    # `i` must be non-negative and less than `x.word_count`
-    d = x.d
-    (GC.@preserve d unsafe_load(Ptr{T}(pointer(d)), i + 1))::T
+function get_elem(x::RawBigInt, i::Int, ::Val{:words}, ::Val{:ascending})
+    @inbounds @inline get_elem_words_raw(x, i)
 end
 
 function get_elem(x, i::Int, v::Val, ::Val{:descending})
@@ -96,25 +103,31 @@ end
 
 """
 Returns an integer of type `R`, consisting of the `len` most
-significant bits of `x`.
+significant bits of `x`. If there are less than `len` bits in `x`,
+the least significant bits are zeroed.
 """
 function truncated(::Type{R}, x::RawBigInt, len::Int) where {R<:Integer}
     ret = zero(R)
     if 0 < len
         word_count, bit_count_in_word = split_bit_index(x, len)
         k = word_length(x)
         vals = (Val(:words), Val(:descending))
+        lenx = elem_count(x, first(vals))
 
         for w ∈ 0:(word_count - 1)
             ret <<= k
-            word = get_elem(x, w, vals...)
-            ret |= R(word)
+            if w < lenx
+                word = get_elem(x, w, vals...)
+                ret |= R(word)
+            end
         end
 
         if !iszero(bit_count_in_word)
             ret <<= bit_count_in_word
-            wrd = get_elem(x, word_count, vals...)
-            ret |= R(wrd >>> (k - bit_count_in_word))
+            if word_count < lenx
+                wrd = get_elem(x, word_count, vals...)
+                ret |= R(wrd >>> (k - bit_count_in_word))
+            end
         end
     end
     ret::R

test/mpfr.jl

@@ -1088,3 +1088,12 @@ end
         clear_flags()
     end
 end
+
+@testset "RawBigInt truncation OOB read" begin
+    @testset "T: $T" for T ∈ (UInt8, UInt16, UInt32, UInt64, UInt128)
+        v = Base.RawBigInt{T}("a"^sizeof(T), 1)
+        @testset "bit_count: $bit_count" for bit_count ∈ (0:10:80)
+            @test Base.truncated(UInt128, v, bit_count) isa Any
+        end
+    end
+end