elf: define segment indexing function

Returns a pointer to the segment in the table at the specified index.
Checks that the index is within bounds and determines the correct offset
given the structural parameters of the input ELF.

include/lone/elf.h

@@ -772,6 +772,8 @@ bool lone_elf_header_machine_is_reserved(lone_u16 machine);
 
 struct lone_elf_segments lone_elf_header_read_segments(struct lone_elf_header *header);
 
+struct lone_elf_segment *lone_elf_segment_at(struct lone_elf_segments segments, lone_u16 index);
+
 /* ╭────────────────────────────────────────────────────────────────────────╮
    │                                                                        │
    │    Functions for validating an ELF header as a whole or in parts.      │

source/lone/elf.c

@@ -254,6 +254,30 @@ lone_elf_header_read_segments(struct lone_elf_header *header)
 	};
 }
 
+struct lone_elf_segment *
+lone_elf_segment_at(struct lone_elf_segments table, lone_u16 index)
+{
+	uintptr_t address;
+	size_t offset;
+
+	if (!table.segment.size  ||
+	    !table.segment.count ||
+	    !table.segments      ||
+	     index >= table.segment.count) { return 0; }
+
+	address = (uintptr_t) table.segments;
+
+	if (__builtin_mul_overflow(index, table.segment.size, &offset)) {
+		return 0;
+	}
+
+	if (__builtin_add_overflow(address, offset, &address)) {
+		return 0;
+	}
+
+	return (struct lone_elf_segment *) address;
+}
+
 /* ╭────────────────────────────────────────────────────────────────────────╮
    │                                                                        │
    │    Writers for ELF data structures.                                    │