[PATCH v2 1/3] x86/mkelf32: tidy what is written out

[Jan Beulich <jbeulich@xxxxxxxx>]

First, replace an open-coded literal number by an expression.

Then adjust tail padding: Don't write up to 4 bytes (and at least one),
but truly only pad to the next 4-byte boundary. And pad with zeroes, not
with whatever is left in buffer[].

Finally drop the explicit nul character in out_shstrtab_extra[] - the one
which is being appended anyway will do fine.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v2: New.

--- a/xen/arch/x86/boot/mkelf32.c
+++ b/xen/arch/x86/boot/mkelf32.c
@@ -68,7 +68,7 @@ static Elf32_Phdr note_phdr = {
 
 static uint8_t out_shstrtab[] = "\0.text\0.shstrtab";
 /* If num_phdrs >= 2, we need to tack the .note. */
-static uint8_t out_shstrtab_extra[] = ".note\0";
+static uint8_t out_shstrtab_extra[] = ".note";
 
 static Elf32_Shdr out_shdr[] = {
     { 0 },
@@ -97,11 +97,11 @@ static Elf32_Shdr out_shdr[] = {
 };
 
 /*
- * The 17 points to the '.note' in the out_shstrtab and out_shstrtab_extra
+ * .sh_name points to the '.note' in the out_shstrtab and out_shstrtab_extra
  * laid out in the file.
  */
 static Elf32_Shdr out_shdr_note = {
-      17,                                    /* sh_name */
+      sizeof(out_shstrtab),                  /* sh_name */
       SHT_NOTE,                              /* sh_type */
       0,                                     /* sh_flags */
       DYNAMICALLY_FILLED,                    /* sh_addr */
@@ -468,13 +468,21 @@ int main(int argc, char **argv)
         do_write(outfd, out_shstrtab, sizeof(out_shstrtab));
         /* Our .note */
         do_write(outfd, out_shstrtab_extra, sizeof(out_shstrtab_extra));
-        do_write(outfd, buffer, 
4-((sizeof(out_shstrtab)+sizeof(out_shstrtab_extra)+dat_siz)&3));
+        bytes = sizeof(out_shstrtab) + sizeof(out_shstrtab_extra) + dat_siz;
     }
     else
     {
         do_write(outfd, out_shstrtab, sizeof(out_shstrtab));
-        do_write(outfd, buffer, 4-((sizeof(out_shstrtab)+dat_siz)&3));
+        bytes = sizeof(out_shstrtab) + dat_siz;
     }
+
+    /* Pad to a multiple of 4 bytes, whatever that's good for. */
+    if ( bytes & 3 )
+    {
+        memset(buffer, 0, 4);
+        do_write(outfd, buffer, 4 - (bytes & 3));
+    }
+
     close(infd);
     close(outfd);