Optimizing Struct Field Alignment

[ZuhairORZaki]

Overview

In file: db.go, the fields of the Options struct are organized in a way that takes more memory. The fields of the struct can be sorted in a way that takes less memory than the initial alignment.

// Options represents the options that can be set when opening a database.
type Options struct {
	// Timeout is the amount of time to wait to obtain a file lock.
	// When set to zero it will wait indefinitely.
	Timeout time.Duration

	// Sets the DB.NoGrowSync flag before memory mapping the file.
	NoGrowSync bool

	// Do not sync freelist to disk. This improves the database write performance
	// under normal operation, but requires a full database re-sync during recovery.
	NoFreelistSync bool

	// PreLoadFreelist sets whether to load the free pages when opening
	// the db file. Note when opening db in write mode, bbolt will always
	// load the free pages.
	PreLoadFreelist bool

	// FreelistType sets the backend freelist type. There are two options. Array which is simple but endures
	// dramatic performance degradation if database is large and fragmentation in freelist is common.
	// The alternative one is using hashmap, it is faster in almost all circumstances
	// but it doesn't guarantee that it offers the smallest page id available. In normal case it is safe.
	// The default type is array
	FreelistType FreelistType

	// Open database in read-only mode. Uses flock(..., LOCK_SH |LOCK_NB) to
	// grab a shared lock (UNIX).
	ReadOnly bool

	// Sets the DB.MmapFlags flag before memory mapping the file.
	MmapFlags int

	// InitialMmapSize is the initial mmap size of the database
	// in bytes. Read transactions won't block write transaction
	// if the InitialMmapSize is large enough to hold database mmap
	// size. (See DB.Begin for more information)
	//
	// If <=0, the initial map size is 0.
	// If initialMmapSize is smaller than the previous database size,
	// it takes no effect.
	InitialMmapSize int

	// PageSize overrides the default OS page size.
	PageSize int

	// NoSync sets the initial value of DB.NoSync. Normally this can just be
	// set directly on the DB itself when returned from Open(), but this option
	// is useful in APIs which expose Options but not the underlying DB.
	NoSync bool

	// OpenFile is used to open files. It defaults to os.OpenFile. This option
	// is useful for writing hermetic tests.
	OpenFile func(string, int, os.FileMode) (*os.File, error)

	// Mlock locks database file in memory when set to true.
	// It prevents potential page faults, however
	// used memory can't be reclaimed. (UNIX only)
	Mlock bool

	// Logger is the logger used for bbolt.
	Logger Logger
}

For the above alignment, the size of the struct is 104 bytes (for 64-bit systems). It can be reduced to 80 bytes by aligning the fields as follows:

type Options struct {
	// Timeout is the amount of time to wait to obtain a file lock.
	// When set to zero it will wait indefinitely.
	Timeout time.Duration

	// FreelistType sets the backend freelist type. There are two options. Array which is simple but endures
	// dramatic performance degradation if database is large and fragmentation in freelist is common.
	// The alternative one is using hashmap, it is faster in almost all circumstances
	// but it doesn't guarantee that it offers the smallest page id available. In normal case it is safe.
	// The default type is array
	FreelistType FreelistType

	// Sets the DB.MmapFlags flag before memory mapping the file.
	MmapFlags int

	// InitialMmapSize is the initial mmap size of the database
	// in bytes. Read transactions won't block write transaction
	// if the InitialMmapSize is large enough to hold database mmap
	// size. (See DB.Begin for more information)
	//
	// If <=0, the initial map size is 0.
	// If initialMmapSize is smaller than the previous database size,
	// it takes no effect.
	InitialMmapSize int

	// PageSize overrides the default OS page size.
	PageSize int

	// OpenFile is used to open files. It defaults to os.OpenFile. This option
	// is useful for writing hermetic tests.
	OpenFile func(string, int, os.FileMode) (*os.File, error)

	// Logger is the logger used for bbolt.
	Logger Logger

		// Mlock locks database file in memory when set to true.
	// It prevents potential page faults, however
	// used memory can't be reclaimed. (UNIX only)
	Mlock bool

	// Sets the DB.NoGrowSync flag before memory mapping the file.
	NoGrowSync bool

	// Do not sync freelist to disk. This improves the database write performance
	// under normal operation, but requires a full database re-sync during recovery.
	NoFreelistSync bool

	// PreLoadFreelist sets whether to load the free pages when opening
	// the db file. Note when opening db in write mode, bbolt will always
	// load the free pages.
	PreLoadFreelist bool

	// Open database in read-only mode. Uses flock(..., LOCK_SH |LOCK_NB) to
	// grab a shared lock (UNIX).
	ReadOnly bool

	// NoSync sets the initial value of DB.NoSync. Normally this can just be
	// set directly on the DB itself when returned from Open(), but this option
	// is useful in APIs which expose Options but not the underlying DB.
	NoSync bool
}

Sponsorship and Support:

This work is done by the security researchers from OpenRefactory and is supported by the Open Source Security Foundation (OpenSSF): Project Alpha-Omega. Alpha-Omega is a project partnering with open source software project maintainers to systematically find new, as-yet-undiscovered vulnerabilities in open source code - and get them fixed - to improve global software supply chain security.

The bug is found by running the iCR tool by OpenRefactory, Inc. and then manually triaging the results.

[ivanvc]

This is being worked on PR #780.

[ZuhairORZaki]

@ivanvc Thank you for clarifying.
Then I will close this issue.