[PATCH v2 3/3] x86/mkelf32: re-write ELF notes

[Jan Beulich <jbeulich@xxxxxxxx>]

64-bit ELF notes generally (sadly with exceptions) are padded to 8-byte
boundaries, whereas 32-bit ELF notes are padded to 4-byte ones. The excess
padding makes it impossible for tools like objdump to properly deal with
these notes.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Is there a need to further generalize or tighten anything?
---
v2: New.

--- a/xen/arch/x86/boot/mkelf32.c
+++ b/xen/arch/x86/boot/mkelf32.c
@@ -8,6 +8,7 @@
  */
 
 #include <errno.h>
+#include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -256,6 +257,8 @@ int main(int argc, char **argv)
     char       buffer[1024] = {};
     int        bytes, todo, i = 1;
     int        num_phdrs = 1;
+    void      *notes32 = NULL;
+    unsigned int note_sz32 = 0;
 
     Elf32_Ehdr in32_ehdr;
 
@@ -359,8 +362,6 @@ int main(int argc, char **argv)
         do_read(infd, &in64_phdr, sizeof(in64_phdr));
         endianadjust_phdr64(&in64_phdr);
 
-        (void)lseek(infd, offset, SEEK_SET);
-
         note_sz = in64_phdr.p_memsz;
         note_base = in64_phdr.p_vaddr - note_base;
 
@@ -373,6 +374,84 @@ int main(int argc, char **argv)
                     offset, offset + dat_siz);
             return 1;
         }
+
+        note_sz32 = note_sz;
+
+        /* Convert 8-byte padded notes to 4-byte padded ones. */
+        if ( in64_phdr.p_align == 8 )
+        {
+            Elf64_Note *notes64 = malloc(note_sz);
+
+            notes32 = malloc(note_sz);
+            if ( notes64 && notes32 )
+            {
+                unsigned int left = note_sz;
+                void *ptr32 = notes32;
+
+                lseek(infd, in64_phdr.p_offset, SEEK_SET);
+                do_read(infd, notes64, note_sz);
+
+                for ( bool start = true; left > sizeof(*notes64); )
+                {
+                    unsigned int size;
+
+                    if ( start && !notes64->namesz )
+                    {
+                        /* Padding. */
+                        notes64 = (void *)(&notes64->namesz + 1);
+                        left -= sizeof(notes64->namesz);
+                        start = false;
+                        continue;
+                    }
+
+                    /*
+                     * The note descriptor may start at either the next 4- or
+                     * 8-byte boundary.  See e.g.
+                     * https://sourceware.org/bugzilla/show_bug.cgi?id=33259.
+                     * For the notes we have actively in use at the time of
+                     * writing, rounding to the next 4-byte boundary is enough
+                     * (and in fact is already excessive: "GNU" and "Xen" as
+                     * note names both satisfy the 8-byte alignment without
+                     * extra effort).
+                     *
+                     * The similar ->descsz aspect is covered by the check
+                     * above.
+                     */
+                    size = ROUNDUP(sizeof(*notes64) + notes64->namesz, 4) +
+                           ROUNDUP(notes64->descsz, 4);
+                    if ( left < size )
+                    {
+                        fprintf(stderr,
+                                "Warning: ELF note overrunning segment (%u 
bytes left, %u bytes claimed)\n",
+                                left, size);
+                        left = 0;
+                        break;
+                    }
+
+                    /* Leverage Elf{32,64}_Note actually having same layout. */
+                    memcpy(ptr32, notes64, size);
+
+                    notes64 = (void *)notes64 + size;
+                    ptr32 += size;
+                    left -= size;
+                    start = true;
+                }
+
+                if ( left )
+                    fprintf(stderr, "Warning: %u trailing bytes of ELF 
notes\n",
+                            left);
+
+                note_sz32 = ptr32 - notes32;
+                memset(ptr32, 0, note_sz - note_sz32);
+            }
+            else
+                fprintf(stderr,
+                        "Warning: Not enough memory to re-write %"PRIu32" 
bytes of ELF notes\n",
+                        note_sz);
+        }
+
+        (void)lseek(infd, offset, SEEK_SET);
+
         /* Gets us the absolute offset within the .text section. */
         offset = in64_phdr.p_offset - offset;
     }
@@ -405,8 +484,8 @@ int main(int argc, char **argv)
         /* Fill out the PT_NOTE program header. */
         note_phdr.p_vaddr   = note_base;
         note_phdr.p_paddr   = note_base;
-        note_phdr.p_filesz  = note_sz;
-        note_phdr.p_memsz   = note_sz;
+        note_phdr.p_filesz  = note_sz32;
+        note_phdr.p_memsz   = note_sz32;
         note_phdr.p_offset  = RAW_OFFSET + offset;
 
         /* Tack on the .note\0 */
@@ -415,7 +494,7 @@ int main(int argc, char **argv)
         out_shdr[2].sh_offset += sizeof(out_shdr_note);
 
         /* Fill out the .note section. */
-        out_shdr_note.sh_size = note_sz;
+        out_shdr_note.sh_size = note_sz32;
         out_shdr_note.sh_addr = note_base;
         out_shdr_note.sh_offset = RAW_OFFSET + offset;
     }
@@ -483,6 +562,17 @@ int main(int argc, char **argv)
         do_write(outfd, buffer, 4 - (bytes & 3));
     }
 
+    if ( notes32 )
+    {
+        lseek(outfd, note_phdr.p_offset, SEEK_SET);
+
+        /*
+         * While we use note_sz32 above, overwrite the full original contents,
+         * to not leave confusing rubbish there.
+         */
+        do_write(outfd, notes32, note_sz);
+    }
+
     close(infd);
     close(outfd);