syscallcompat: add Fgetxattr / Fsetxattr wrappers

These take care of buffer sizing and parsing.
This commit is contained in:
Jakob Unterwurzacher 2019-01-02 16:56:23 +01:00
parent 55a27a47df
commit 7995a8358e
1 changed files with 67 additions and 0 deletions

View File

@ -1,6 +1,7 @@
package syscallcompat
import (
"bytes"
"syscall"
"golang.org/x/sys/unix"
@ -45,3 +46,69 @@ func Faccessat(dirfd int, path string, mode uint32) error {
func Linkat(olddirfd int, oldpath string, newdirfd int, newpath string, flags int) (err error) {
return unix.Linkat(olddirfd, oldpath, newdirfd, newpath, flags)
}
const XATTR_SIZE_MAX = 65536
// Make the buffer 1kB bigger so we can detect overflows
const XATTR_BUFSZ = XATTR_SIZE_MAX + 1024
// Fgetxattr is a wrapper around unix.Fgetxattr that handles the buffer sizing.
func Fgetxattr(fd int, attr string) (val []byte, err error) {
// If the buffer is too small to fit the value, Linux and MacOS react
// differently:
// Linux: returns an ERANGE error and "-1" bytes.
// MacOS: truncates the value and returns "size" bytes.
//
// We choose the simple approach of buffer that is bigger than the limit on
// Linux, and return an error for everything that is bigger (which can
// only happen on MacOS).
//
// See https://github.com/pkg/xattr for a smarter solution.
// TODO: be smarter?
buf := make([]byte, XATTR_BUFSZ)
sz, err := unix.Fgetxattr(fd, attr, buf)
if err == syscall.ERANGE {
// Do NOT return ERANGE - the user might retry ad inifinitum!
return nil, syscall.EOVERFLOW
}
if err != nil {
return nil, err
}
if sz >= XATTR_SIZE_MAX {
return nil, syscall.EOVERFLOW
}
// Copy only the actually used bytes to a new (smaller) buffer
// so "buf" never leaves the function and can be allocated on the stack.
val = make([]byte, sz)
copy(val, buf)
return val, nil
}
// Flistxattr is a wrapper unix.Flistxattr that handles buffer sizing and
// parsing the returned blob to a string slice.
func Flistxattr(fd int) (attrs []string, err error) {
// See the buffer sizing comments in Fgetxattr.
// TODO: be smarter?
buf := make([]byte, XATTR_BUFSZ)
sz, err := unix.Flistxattr(fd, buf)
if err == syscall.ERANGE {
// Do NOT return ERANGE - the user might retry ad inifinitum!
return nil, syscall.EOVERFLOW
}
if err != nil {
return nil, err
}
if sz >= XATTR_SIZE_MAX {
return nil, syscall.EOVERFLOW
}
buf = buf[:sz]
parts := bytes.Split(buf, []byte{0})
for _, part := range parts {
if len(part) == 0 {
// Last part is empty, ignore
continue
}
attrs = append(attrs, string(part))
}
return attrs, nil
}