RtlCreateAcl function (ntifs.h)

Article
02/22/2023

The RtlCreateAcl routine creates and initializes an access control list (ACL).

Syntax

NTSYSAPI NTSTATUS RtlCreateAcl(
  [out] PACL  Acl,
  [in]  ULONG AclLength,
        ULONG AclRevision
);

Parameters

[out] Acl

Pointer to a caller-allocated buffer to receive the initialized ACL structure. This buffer must be at least sizeof(ACL),

[in] AclLength

Length, in bytes, of the buffer pointed to by the Acl parameter. This value must be large enough to contain the ACL header and all of the access-control entries (ACE) to be stored in the ACL. See the following Remarks section for information about calculating the size of an ACL.

AclRevision

ACL revision level of the ACE to be added. Windows version requirements are the following:

Value	Meaning
ACL_REVISION	The revision level valid on all Windows versions.
ACL_REVISION_DS	Note AceRevision must be ACL_REVISION_DS if the ACL in Acl contains an object-specific ACE.

Return value

RtlCreateAcl can return one of the following status values:

Return code	Description
STATUS_SUCCESS	The ACL was successfully created and initialized.
STATUS_BUFFER_TOO_SMALL	The new ACL does not fit into the buffer at Acl. A larger ACL buffer is required.
STATUS_INVALID_PARAMETER	The specified revision is not current, or the value of AclLength is too large.

Remarks

The ACL that is initialized by RtlCreateAcl contains no access control entries (ACE). This ACL is empty, as opposed to being a nonexistent ACL. If an empty ACL is applied to an object, the ACL implicitly denies all access to that object. To add ACEs to the ACL, use RtlAddAccessAllowedAce.

To calculate the size of an ACL, add sizeof(ACL) to the size of all the ACEs to be stored in the ACL. To calculate the size of an ACE, add the size of the ACE structure, such as sizeof(ACCESS_ALLOWED_ACE), to the length of the SID associated with the ACE, and then subtract the size of the SidStart member (which is part of both the ACE structure and the SID). Use the RtlLengthSid function to get the length of a specified SID.

The following example shows how to calculate the size of an access-allowed ACE:

sizeof (ACCESS_ALLOWED_ACE) - sizeof (ACCESS_ALLOWED_ACE.SidStart) 
        + GetLengthSid (pAceSid);

To calculate the size of an ACL, use the following algorithm, substituting the appropriate ACE structure in the sizeof(ACE) expression:

cbAcl = sizeof (ACL);
for (i = 0 ; i < nAceCount ; i++) {
    // subtract ACE.SidStart from the size
    cbAce = sizeof (ACE) - sizeof (DWORD);
    // add this ACE's SID length
    cbAce += GetLengthSid (pAceSid[i]);
    // add the length of each ACE to the total ACL length
    cbAcl += cbAce;
}

For more information about security and access control, see Windows security model for driver developers and the documentation on these topics in the Windows SDK.

Requirements

Requirement	Value
Target Platform	Universal
Header	ntifs.h (include Ntifs.h)
Library	NtosKrnl.lib
DLL	NtosKrnl.exe
IRQL	<= APC_LEVEL