lib: fs: stop using libbtrfs-dev helper functions to re-enable buffer length checks

The Linux kernel's btrfs headers are better than the libbtrfs-dev headers:

	- the libbtrfs-dev headers have C++ language compatibility issues

	- upstream version in Linux kernel is more accurate and up to date

	- macros in libbtrfs-dev's ctree.h hide information that would
	enable bees to perform runtime buffer length checking

	- enum types whose presence cannot be detected with #ifdef

When accessing members of metadata items from the filesystem, we want
to verify that the member we are accessing is within the boundaries of
the item that was retrieved; otherwise, a memory access violation may
occur or garbage may be returned to the caller.  A simple C++ template,
given a pointer to a structure member and a buffer, can determine that
the buffer contains enough bytes to safely access a struct member.
This was implemented back in 2016, but left unused due to ctree.h issues.

Some btrfs metadata structures have variable length despite using a
fixed-size in-memory structure.  The members that appear earliest in
the structure contain information about which following members of the
structure are used.  The item stored in the filesystem is truncated after
the last used member, and all following members must not be accessed.

'btrfs_stack_*' accessor macros obscure the memory boundaries of the
members they access, which makes it impossible for a C++ template to
verify the memory access.  If the template checks the length of the
entire structure, it will find an access violation for variable-length
metadata items because the item is rarely large enough for the entire
structure.

Get rid of all the libbtrfs-dev accessor macros and reimplement them
with the necessary buffer length checks.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

include/crucible/fs.h

@@ -1,6 +1,7 @@
 #ifndef CRUCIBLE_FS_H
 #define CRUCIBLE_FS_H
 
+#include "crucible/endian.h"
 #include "crucible/error.h"
 #include "crucible/spanner.h"
 
@@ -206,47 +207,21 @@ namespace crucible {
 	const T*
 	get_struct_ptr(const V &v, size_t offset = 0)
 	{
-		// OK so sometimes btrfs overshoots a little
-		// if (offset + sizeof(T) > v.size()) {
-			// v.resize(offset + sizeof(T), 0);
-		// }
-		// THROW_CHECK2(invalid_argument, v.size(), offset + sizeof(T), offset + sizeof(T) <= v.size());
-		return reinterpret_cast<const T*>(v.data() + offset);
+		THROW_CHECK2(out_of_range, v.size(), offset + sizeof(T), offset + sizeof(T) <= v.size());
+		const uint8_t *const data_ptr = v.data();
+		return reinterpret_cast<const T*>(data_ptr + offset);
 	}
 
-	template<class A, class R, class V>
-	R
-	call_btrfs_get(R (*func)(const A*), const V &v, size_t offset = 0)
-	{
-		return func(get_struct_ptr<A, V>(v, offset));
-	}
-
-	template <class T> struct btrfs_get_le;
-
-	template<> struct btrfs_get_le<__le64> {
-		uint64_t operator()(const void *p) { return get_unaligned_le64(p); }
-	};
-
-	template<> struct btrfs_get_le<__le32> {
-		uint32_t operator()(const void *p) { return get_unaligned_le32(p); }
-	};
-
-	template<> struct btrfs_get_le<__le16> {
-		uint16_t operator()(const void *p) { return get_unaligned_le16(p); }
-	};
-
-	template<> struct btrfs_get_le<__le8> {
-		uint8_t operator()(const void *p) { return get_unaligned_le8(p); }
-	};
-
 	template<class S, class T, class V>
 	T
 	btrfs_get_member(T S::* member, V &v, size_t offset = 0)
 	{
 		const S *const sp = nullptr;
 		const T *const spm = &(sp->*member);
-		auto member_offset = reinterpret_cast<const uint8_t *>(spm) - reinterpret_cast<const uint8_t *>(sp);
-		return btrfs_get_le<T>()(get_struct_ptr<S>(v, offset + member_offset));
+		const auto member_offset = reinterpret_cast<const uint8_t *>(spm) - reinterpret_cast<const uint8_t *>(sp);
+		const void *struct_ptr = get_struct_ptr<T>(v, offset + member_offset);
+		const T unaligned_t = get_unaligned<T>(struct_ptr);
+		return le_to_cpu(unaligned_t);
 	}
 
 	struct Statvfs : public statvfs {