203 lines
7.0 KiB
Go
203 lines
7.0 KiB
Go
// Copyright 2019 The gVisor Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package vfs
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import (
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"gvisor.dev/gvisor/pkg/abi/linux"
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"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
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"gvisor.dev/gvisor/pkg/syserror"
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)
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// AccessTypes is a bitmask of Unix file permissions.
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type AccessTypes uint16
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// Bits in AccessTypes.
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const (
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MayRead AccessTypes = 4
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MayWrite = 2
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MayExec = 1
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)
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// OnlyRead returns true if access _only_ allows read.
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func (a AccessTypes) OnlyRead() bool {
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return a == MayRead
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}
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// MayRead returns true if access allows read.
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func (a AccessTypes) MayRead() bool {
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return a&MayRead != 0
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}
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// MayWrite returns true if access allows write.
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func (a AccessTypes) MayWrite() bool {
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return a&MayWrite != 0
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}
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// MayExec returns true if access allows exec.
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func (a AccessTypes) MayExec() bool {
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return a&MayExec != 0
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}
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// GenericCheckPermissions checks that creds has the given access rights on a
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// file with the given permissions, UID, and GID, subject to the rules of
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// fs/namei.c:generic_permission(). isDir is true if the file is a directory.
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func GenericCheckPermissions(creds *auth.Credentials, ats AccessTypes, isDir bool, mode uint16, kuid auth.KUID, kgid auth.KGID) error {
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// Check permission bits.
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perms := mode
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if creds.EffectiveKUID == kuid {
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perms >>= 6
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} else if creds.InGroup(kgid) {
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perms >>= 3
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}
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if uint16(ats)&perms == uint16(ats) {
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return nil
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}
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// Caller capabilities require that the file's KUID and KGID are mapped in
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// the caller's user namespace; compare
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// kernel/capability.c:privileged_wrt_inode_uidgid().
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if !kuid.In(creds.UserNamespace).Ok() || !kgid.In(creds.UserNamespace).Ok() {
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return syserror.EACCES
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}
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// CAP_DAC_READ_SEARCH allows the caller to read and search arbitrary
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// directories, and read arbitrary non-directory files.
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if (isDir && !ats.MayWrite()) || ats.OnlyRead() {
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if creds.HasCapability(linux.CAP_DAC_READ_SEARCH) {
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return nil
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}
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}
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// CAP_DAC_OVERRIDE allows arbitrary access to directories, read/write
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// access to non-directory files, and execute access to non-directory files
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// for which at least one execute bit is set.
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if isDir || !ats.MayExec() || (mode&0111 != 0) {
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if creds.HasCapability(linux.CAP_DAC_OVERRIDE) {
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return nil
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}
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}
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return syserror.EACCES
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}
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// AccessTypesForOpenFlags returns the access types required to open a file
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// with the given OpenOptions.Flags. Note that this is NOT the same thing as
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// the set of accesses permitted for the opened file:
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//
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// - O_TRUNC causes MayWrite to be set in the returned AccessTypes (since it
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// mutates the file), but does not permit writing to the open file description
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// thereafter.
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//
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// - "Linux reserves the special, nonstandard access mode 3 (binary 11) in
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// flags to mean: check for read and write permission on the file and return a
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// file descriptor that can't be used for reading or writing." - open(2). Thus
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// AccessTypesForOpenFlags returns MayRead|MayWrite in this case.
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//
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// Use May{Read,Write}FileWithOpenFlags() for these checks instead.
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func AccessTypesForOpenFlags(flags uint32) AccessTypes {
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switch flags & linux.O_ACCMODE {
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case linux.O_RDONLY:
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if flags&linux.O_TRUNC != 0 {
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return MayRead | MayWrite
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}
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return MayRead
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case linux.O_WRONLY:
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return MayWrite
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default:
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return MayRead | MayWrite
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}
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}
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// MayReadFileWithOpenFlags returns true if a file with the given open flags
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// should be readable.
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func MayReadFileWithOpenFlags(flags uint32) bool {
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switch flags & linux.O_ACCMODE {
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case linux.O_RDONLY, linux.O_RDWR:
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return true
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default:
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return false
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}
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}
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// MayWriteFileWithOpenFlags returns true if a file with the given open flags
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// should be writable.
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func MayWriteFileWithOpenFlags(flags uint32) bool {
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switch flags & linux.O_ACCMODE {
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case linux.O_WRONLY, linux.O_RDWR:
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return true
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default:
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return false
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}
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}
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// CheckSetStat checks that creds has permission to change the metadata of a
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// file with the given permissions, UID, and GID as specified by stat, subject
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// to the rules of Linux's fs/attr.c:setattr_prepare().
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func CheckSetStat(creds *auth.Credentials, stat *linux.Statx, mode uint16, kuid auth.KUID, kgid auth.KGID) error {
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if stat.Mask&linux.STATX_MODE != 0 {
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if !CanActAsOwner(creds, kuid) {
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return syserror.EPERM
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}
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// TODO(b/30815691): "If the calling process is not privileged (Linux:
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// does not have the CAP_FSETID capability), and the group of the file
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// does not match the effective group ID of the process or one of its
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// supplementary group IDs, the S_ISGID bit will be turned off, but
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// this will not cause an error to be returned." - chmod(2)
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}
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if stat.Mask&linux.STATX_UID != 0 {
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if !((creds.EffectiveKUID == kuid && auth.KUID(stat.UID) == kuid) ||
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HasCapabilityOnFile(creds, linux.CAP_CHOWN, kuid, kgid)) {
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return syserror.EPERM
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}
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}
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if stat.Mask&linux.STATX_GID != 0 {
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if !((creds.EffectiveKUID == kuid && creds.InGroup(auth.KGID(stat.GID))) ||
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HasCapabilityOnFile(creds, linux.CAP_CHOWN, kuid, kgid)) {
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return syserror.EPERM
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}
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}
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if stat.Mask&(linux.STATX_ATIME|linux.STATX_MTIME|linux.STATX_CTIME) != 0 {
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if !CanActAsOwner(creds, kuid) {
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if (stat.Mask&linux.STATX_ATIME != 0 && stat.Atime.Nsec != linux.UTIME_NOW) ||
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(stat.Mask&linux.STATX_MTIME != 0 && stat.Mtime.Nsec != linux.UTIME_NOW) ||
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(stat.Mask&linux.STATX_CTIME != 0 && stat.Ctime.Nsec != linux.UTIME_NOW) {
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return syserror.EPERM
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}
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// isDir is irrelevant in the following call to
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// GenericCheckPermissions since ats == MayWrite means that
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// CAP_DAC_READ_SEARCH does not apply, and CAP_DAC_OVERRIDE
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// applies, regardless of isDir.
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if err := GenericCheckPermissions(creds, MayWrite, false /* isDir */, mode, kuid, kgid); err != nil {
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return err
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}
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}
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}
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return nil
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}
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// CanActAsOwner returns true if creds can act as the owner of a file with the
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// given owning UID, consistent with Linux's
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// fs/inode.c:inode_owner_or_capable().
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func CanActAsOwner(creds *auth.Credentials, kuid auth.KUID) bool {
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if creds.EffectiveKUID == kuid {
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return true
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}
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return creds.HasCapability(linux.CAP_FOWNER) && creds.UserNamespace.MapFromKUID(kuid).Ok()
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}
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// HasCapabilityOnFile returns true if creds has the given capability with
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// respect to a file with the given owning UID and GID, consistent with Linux's
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// kernel/capability.c:capable_wrt_inode_uidgid().
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func HasCapabilityOnFile(creds *auth.Credentials, cp linux.Capability, kuid auth.KUID, kgid auth.KGID) bool {
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return creds.HasCapability(cp) && creds.UserNamespace.MapFromKUID(kuid).Ok() && creds.UserNamespace.MapFromKGID(kgid).Ok()
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}
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