Clean up and apply the fix for PR 83477. The calculation for locating

the start of the section headers has to take into account the fact
that the image_nt_header is really variable sized. It happens that
the existing calculation is correct for _most_ production binaries
produced by the Windows DDK, but if we get a binary with oddball
offsets, the PE loader could crash.

Changes from the supplied patch are:

- We don't really need to use the IMAGE_SIZEOF_NT_HEADER() macro when
  computing how much of the header to return to callers of
  pe_get_optional_header(). While it's important to take the variable
  size of the header into account in other calculations, we never
  actually look at anything outside the non-variable portion of the
  header. This saves callers from having to allocate a variable sized
  buffer off the heap (I purposely tried to avoid using malloc()
  in subr_pe.c to make it easier to compile in both the -D_KERNEL and
  !-D_KERNEL case), and since we're copying into a buffer on the
  stack, we always have to copy the same amount of data or else
  we'll trash the stack something fierce.

- We need <stddef.h> to get offsetof() in the !-D_KERNEL case.

- ndiscvt.c needs the IMAGE_FIRST_SECTION() macro too, since it does
  a little bit of section pre-processing.

PR: kern/83477

sys/compat/ndis/pe_var.h

@@ -214,6 +214,10 @@ struct image_nt_header {
 
 typedef struct image_nt_header image_nt_header;
 
+#define	IMAGE_SIZEOF_NT_HEADER(nthdr)					\
+	(offsetof(image_nt_header, inh_optionalhdr) +			\
+	  ((image_nt_header *)(nthdr))->inh_filehdr.ifh_optionalhdrlen)
+
 /* Directory Entries */
 
 #define IMAGE_DIRECTORY_ENTRY_EXPORT         0   /* Export Directory */
@@ -281,6 +285,11 @@ typedef struct image_section_header image_section_header;
 
 #define IMAGE_SIZEOF_SECTION_HEADER          40
 
+#define IMAGE_FIRST_SECTION(nthdr)					\
+	((image_section_header *)((vm_offset_t)(nthdr) +		\
+	  offsetof(image_nt_header, inh_optionalhdr) +			\
+	  ((image_nt_header *)(nthdr))->inh_filehdr.ifh_optionalhdrlen))
+
 /*
  * Import format
  */

sys/compat/ndis/subr_pe.c

@@ -57,6 +57,7 @@ extern int ndis_strncasecmp(const char *, const char *, size_t);
 #define strncasecmp(a, b, c) ndis_strncasecmp(a, b, c)
 #else
 #include <stdio.h>
+#include <stddef.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <string.h>
@@ -142,7 +143,7 @@ pe_get_optional_header(imgbase, hdr)
 	nt_hdr = (image_nt_header *)(imgbase + dos_hdr->idh_lfanew);
 
 	bcopy ((char *)&nt_hdr->inh_optionalhdr, (char *)hdr,
-	    sizeof(image_optional_header));
+	    nt_hdr->inh_filehdr.ifh_optionalhdrlen);
 
 	return(0);
 }
@@ -169,6 +170,14 @@ pe_get_file_header(imgbase, hdr)
 	dos_hdr = (image_dos_header *)imgbase;
 	nt_hdr = (image_nt_header *)(imgbase + dos_hdr->idh_lfanew);
 
+	/*
+	 * Note: the size of the nt_header is variable since it
+	 * can contain optional fields, as indicated by ifh_optionalhdrlen.
+	 * However it happens we're only interested in fields in the
+	 * non-variant portion of the nt_header structure, so we don't
+	 * bother copying the optional parts here.
+	 */
+
 	bcopy ((char *)&nt_hdr->inh_filehdr, (char *)hdr,
 	    sizeof(image_file_header));
 
@@ -197,8 +206,7 @@ pe_get_section_header(imgbase, hdr)
 
 	dos_hdr = (image_dos_header *)imgbase;
 	nt_hdr = (image_nt_header *)(imgbase + dos_hdr->idh_lfanew);
-	sect_hdr = (image_section_header *)((vm_offset_t)nt_hdr +
-	    sizeof(image_nt_header));
+	sect_hdr = IMAGE_FIRST_SECTION(nt_hdr);
 
 	bcopy ((char *)sect_hdr, (char *)hdr, sizeof(image_section_header));
 
@@ -280,8 +288,7 @@ pe_translate_addr(imgbase, rva)
 
 	dos_hdr = (image_dos_header *)imgbase;
 	nt_hdr = (image_nt_header *)(imgbase + dos_hdr->idh_lfanew);
-	sect_hdr = (image_section_header *)((vm_offset_t)nt_hdr +
-	    sizeof(image_nt_header));
+	sect_hdr = IMAGE_FIRST_SECTION(nt_hdr);
 
 	/*
 	 * The test here is to see if the RVA falls somewhere
@@ -339,8 +346,7 @@ pe_get_section(imgbase, hdr, name)
 
 	dos_hdr = (image_dos_header *)imgbase;
 	nt_hdr = (image_nt_header *)(imgbase + dos_hdr->idh_lfanew);
-	sect_hdr = (image_section_header *)((vm_offset_t)nt_hdr +
-	    sizeof(image_nt_header));
+	sect_hdr = IMAGE_FIRST_SECTION(nt_hdr);
 
 	for (i = 0; i < sections; i++) {
 		if (!strcmp ((char *)&sect_hdr->ish_name, name)) {

usr.sbin/ndiscvt/ndiscvt.c

@@ -38,6 +38,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/socket.h>
 #include <net/if.h>
 #include <stdlib.h>
+#include <stddef.h>
 #include <unistd.h>
 #include <stdio.h>
 #include <errno.h>
@@ -88,8 +89,7 @@ extern const char *__progname;
 #define SET_HDRS(x)	\
 	dos_hdr = (image_dos_header *)x;				\
 	nt_hdr = (image_nt_header *)(x + dos_hdr->idh_lfanew);		\
-	sect_hdr = (image_section_header *)((vm_offset_t)nt_hdr +	\
-	    sizeof(image_nt_header));
+	sect_hdr = IMAGE_FIRST_SECTION(nt_hdr);
 
 static
 int insert_padding(imgbase, imglen)