get*ent: be consistant about _ALIGN(p) - p

Add an nscache specific inline function to calculate the misalignment
rather than adding and subtracting _ALIGN(p) and p which can take the
buffer far out of bound (undefined behavior in C and unsupported on
CHERI).

Reviewed by:	kib
Effort:		CHERI upstreaming
Obtained from:	CheriBSD
Sponsored by:	DARPA
Differential Revision:	https://reviews.freebsd.org/D53945

lib/libc/gen/getgrent.c

@@ -347,16 +347,16 @@ grp_unmarshal_func(char *buffer, size_t buffer_size, void *retval, va_list ap,
 	memcpy(&p, buffer + sizeof(struct group), sizeof(char *));
 
 	if (orig_buf_size + sizeof(struct group) + sizeof(char *) +
-	    _ALIGN(p) - (size_t)p < buffer_size) {
+	    __nss_buf_misalignment(p) < buffer_size) {
 		*ret_errno = ERANGE;
 		return (NS_RETURN);
 	}
 
 	orig_buf = (char *)_ALIGN(orig_buf);
 	memcpy(orig_buf, buffer + sizeof(struct group) + sizeof(char *) +
-	    _ALIGN(p) - (size_t)p,
+	    __nss_buf_misalignment(p),
 	    buffer_size - sizeof(struct group) - sizeof(char *) -
-	    _ALIGN(p) + (size_t)p);
+	    __nss_buf_misalignment(p));
 	p = (char *)_ALIGN(p);
 
 	NS_APPLY_OFFSET(grp->gr_name, orig_buf, p, char *);

lib/libc/include/nscache.h

@@ -29,6 +29,8 @@
 #ifndef __NS_CACHE_H__
 #define __NS_CACHE_H__
 
+#include <sys/_align.h>
+
 #include "nscachedcli.h"
 
 typedef int (*nss_cache_id_func_t)(char *, size_t *, va_list, void *);
@@ -178,6 +180,12 @@ typedef struct _nss_cache_data {
 } nss_cache_data;
 
 __BEGIN_DECLS
+static inline __ptrdiff_t
+__nss_buf_misalignment(const void *p)
+{
+	return ((char *)_ALIGN(p) - (char *)p);
+}
+
 /* dummy function, which is needed to make nss_method_lookup happy */
 extern	int	__nss_cache_handler(void *, void *, va_list);
 

lib/libc/net/gethostnamadr.c

@@ -402,9 +402,9 @@ host_unmarshal_func(char *buffer, size_t buffer_size, void *retval, va_list ap,
 
 	orig_buf = (char *)_ALIGN(orig_buf);
 	memcpy(orig_buf, buffer + sizeof(struct hostent) + sizeof(char *) +
-	    _ALIGN(p) - (size_t)p,
+	    __nss_buf_misalignment(p),
 	    buffer_size - sizeof(struct hostent) - sizeof(char *) -
-	    _ALIGN(p) + (size_t)p);
+	    __nss_buf_misalignment(p));
 	p = (char *)_ALIGN(p);
 
 	NS_APPLY_OFFSET(ht->h_name, orig_buf, p, char *);

lib/libc/net/getnetnamadr.c

@@ -249,9 +249,9 @@ net_unmarshal_func(char *buffer, size_t buffer_size, void *retval, va_list ap,
 
 	orig_buf = (char *)_ALIGN(orig_buf);
 	memcpy(orig_buf, buffer + sizeof(struct netent) + sizeof(char *) +
-	    _ALIGN(p) - (size_t)p,
+	    __nss_buf_misalignment(p),
 	    buffer_size - sizeof(struct netent) - sizeof(char *) -
-	    _ALIGN(p) + (size_t)p);
+	    __nss_buf_misalignment(p));
 	p = (char *)_ALIGN(p);
 
 	NS_APPLY_OFFSET(ne->n_name, orig_buf, p, char *);

lib/libc/net/getprotoent.c

@@ -265,9 +265,9 @@ __proto_unmarshal_func(char *buffer, size_t buffer_size, void *retval,
 
 	orig_buf = (char *)_ALIGN(orig_buf);
 	memcpy(orig_buf, buffer + sizeof(struct protoent) + sizeof(char *) +
-	    _ALIGN(p) - (size_t)p,
+	    __nss_buf_misalignment(p),
 	    buffer_size - sizeof(struct protoent) - sizeof(char *) -
-	    _ALIGN(p) + (size_t)p);
+	    __nss_buf_misalignment(p));
 	p = (char *)_ALIGN(p);
 
 	NS_APPLY_OFFSET(proto->p_name, orig_buf, p, char *);

lib/libc/net/getservent.c

@@ -1084,9 +1084,9 @@ serv_unmarshal_func(char *buffer, size_t buffer_size, void *retval, va_list ap,
 
 	orig_buf = (char *)_ALIGN(orig_buf);
 	memcpy(orig_buf, buffer + sizeof(struct servent) + sizeof(char *) +
-	    (_ALIGN(p) - (size_t)p),
+	    __nss_buf_misalignment(p),
 	    buffer_size - sizeof(struct servent) - sizeof(char *) -
-	    (_ALIGN(p) - (size_t)p));
+	    __nss_buf_misalignment(p));
 	p = (char *)_ALIGN(p);
 
 	NS_APPLY_OFFSET(serv->s_name, orig_buf, p, char *);

lib/libc/rpc/getrpcent.c

@@ -778,9 +778,9 @@ rpc_unmarshal_func(char *buffer, size_t buffer_size, void *retval, va_list ap,
 
 	orig_buf = (char *)_ALIGN(orig_buf);
 	memcpy(orig_buf, buffer + sizeof(struct rpcent) + sizeof(char *) +
-	    _ALIGN(p) - (size_t)p,
+	    __nss_buf_misalignment(p),
 	    buffer_size - sizeof(struct rpcent) - sizeof(char *) -
-	    _ALIGN(p) + (size_t)p);
+	    __nss_buf_misalignment(p));
 	p = (char *)_ALIGN(p);
 
 	NS_APPLY_OFFSET(rpc->r_name, orig_buf, p, char *);