Systems ProgrammingUser-Space File System

H. Turgut Uyar Sima Uyar

2009

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Topics

User-Space Development

FUSEIntroductionRead-Only FilesystemHello, world

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System Programming Levels

I compiling the kernel:best performance, every possible functionalityrisky, time-consuming

I kernel modules:very good performance, less risky, fast developmentcan not do everything

I user-space:even less risky, fast development, can use external librariespoorer performance, can not do everything

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FUSE

I Filesystem in Userspace

I develop a file system in user space on top of a kernel module

I non-native filesystems (NTFS, ZFS, ...)I changing data storage (SQL, ...)I providing transparent functionality (compression,

encryption, ...)

I two paths for every file/directory:FUSE path - underlying filesystem path

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FUSE Structure

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FUSE Development

I similar to device driver development:implement system calls

I needed package: libfuse-devI system calls:

I file related:open, release, read, write, getattr, unlink, ...

I directory related:readdir, mkdir, rmdir, ...

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FUSE Development

Example (fuse operations)

s t ruc t f u s e o p e r a t i o n s r o f s o p e r = {. g e t a t t r = r o f s g e t a t t r ,. r e a d d i r = r o f s r e a d d i r ,. mkdir = r o f s m k d i r ,. u n l i n k = r o f s u n l i n k ,. r m d i r = r o f s r m d i r ,. rename = r o f s r e n a m e ,. open = r o f s o p e n ,. r e a d = r o f s r e a d ,. w r i t e = r o f s w r i t e ,. r e l e a s e = r o f s r e l e a s e ,. . .

} ;

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FUSE Development

Example (path translation)

char ∗ rPath = m a l l o c ( s i z eo f ( char )∗( s t r l e n ( path ) + s t r l e n ( rw path ) + 1 ) ) ;

s t r c p y ( rPath , rw path ) ;i f ( rPath [ s t r l e n ( rPath )−1] == ’ / ’ ) {

rPath [ s t r l e n ( rPath )−1] = ’ \0 ’ ;}s t r c a t ( rPath , path ) ;

return rPath ;

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FUSE Development

directory listing: readdir

s t a t i c i n t r o f s r e a d d i r (const char ∗path ,void ∗buf ,f u s e f i l l d i r t f i l l e r ,o f f t o f f s e t ,s t ruc t f u s e f i l e i n f o ∗ f i

) ;

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FUSE Development

Example (directory listing)

upath = t r a n s l a t e p a t h ( path ) ;dp = o p e n d i r ( upath ) ; /∗ DIR ∗dp ; ∗/f r e e ( upath ) ;i f ( dp == NULL) {

r e s = −e r r n o ;return r e s ;

}

/∗ f i l l i n the d i r e c t o r y i n f o ∗/

c l o s e d i r ( dp ) ;

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FUSE Development

Example (directory info)

/∗ s t r u c t d i r e n t ∗de ; ∗/whi le ( ( de = r e a d d i r ( dp ) ) != NULL) {

s t ruc t s t a t s t ;memset(& st , 0 , s i z eo f ( s t ) ) ;s t . s t i n o = de−>d i n o ;s t . st mode = de−>d t y p e << 1 2 ;i f ( f i l l e r ( buf , de−>d name , &st , 0 ) )

break ;}

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FUSE Development

reading file attributes

s t a t i c i n t r o f s g e t a t t r (const char ∗path ,s t ruc t s t a t ∗ s t d a t a

) ;

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FUSE Development

Example (reading file attributes)

upath = t r a n s l a t e p a t h ( path ) ;r e s = l s t a t ( upath , s t d a t a ) ;f r e e ( upath ) ;i f ( r e s == −1) {

return −e r r n o ;}

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FUSE Development

reading from a file

s t a t i c i n t r o f s r e a d (const char ∗path ,char ∗buf ,s i z e t s i z e ,o f f t o f f s e t ,s t ruc t f u s e f i l e i n f o ∗ f i n f o

) ;

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FUSE Development

Example (reading from a file)

upath = t r a n s l a t e p a t h ( path ) ;f d = open ( upath , O RDONLY ) ;f r e e ( upath ) ;i f ( f d == −1) {

r e s = −e r r n o ;return r e s ;

}r e s = pread ( fd , buf , s i z e , o f f s e t ) ;i f ( r e s == −1) {

r e s = −e r r n o ;}c l o s e ( f d ) ;

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FUSE Development

modification operations

s t a t i c i n t r o f s m k d i r (const char ∗path ,mode t mode

) ;

s t a t i c i n t r o f s u n l i n k ( const char ∗ path ) ;

/∗ body ∗/return −EROFS ;

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FUSE Development

I compiling:gcc −o rofs −Wall −ansi −W −std=c99 −g −ggdb−D GNU SOURCE −D FILE OFFSET BITS=64−lfuse rofs .c

I mounting:./ rofs <rw dir> <ro dir>

I unmounting:fusermount −u <ro dir>

I running in debug mode:./ rofs −d <rw dir> <ro dir>

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FUSE Development

Example (fuse operations)

s t a t i c s t ruc t f u s e o p e r a t i o n s h e l l o o p e r = {. g e t a t t r = h e l l o g e t a t t r ,. r e a d d i r = h e l l o r e a d d i r ,. open = h e l l o o p e n ,. r e a d = h e l l o r e a d ,

} ;

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FUSE Development

Example (hello readdir)

i f ( st rcmp ( path , ”/ ”) != 0)return −ENOENT;

f i l l e r ( buf , ” . ” , NULL , 0 ) ;f i l l e r ( buf , ” . . ” , NULL , 0 ) ;f i l l e r ( buf , h e l l o p a t h + 1 , NULL , 0 ) ;

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FUSE Development

Example (hello getattr)

memset ( s t b u f , 0 , s i z eo f ( s t ruc t s t a t ) ) ;i f ( st rcmp ( path , ”/ ”) == 0) {

s t b u f−>st mode = S IFDIR | 0 7 5 5 ;s t b u f−>s t n l i n k = 2 ;

}e l s e i f ( st rcmp ( path , h e l l o p a t h ) == 0) {

s t b u f−>st mode = S IFREG | 0 4 4 4 ;s t b u f−>s t n l i n k = 1 ;s t b u f−>s t s i z e = s t r l e n ( h e l l o s t r ) ;

}e l s e

r e s = −ENOENT;

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FUSE Development

Example (hello read)

i f ( st rcmp ( path , ”/ h e l l o ”) != 0)return −ENOENT;

l e n = s t r l e n ( ”H e l l o , w o r ld !\ n ” ) ;i f ( o f f s e t < l e n ) {

i f ( o f f s e t + s i z e > l e n )s i z e = l e n − o f f s e t ;

memcpy ( buf , ”H e l l o , w o r ld !\ n ”+o f f s e t , s i z e ) ;} e l s e

s i z e = 0 ;

return s i z e ;

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