access phdr header entries like an array

This patch makes it easier and more intuitive to access entries in the phdr header as if it were a C array. It has otherwise no effect on the current code (other than saving some space for an awkward reason), but is needed to implement the ability to load segments at an arbitrary address, distinguishing the relative offset rather than their absolute requested address.

I have the code for all the dance, including the ability to relocate the payload later on, but I've chosen to split it up for revision tracking purposes (besides, the rest needs quite a bit of cleanup yet ;-)).

I've checked there are no regressions (at least with invaders). If there are no objections in a few days I'll check it in.

Signature · 14 lines

Robert Millan

<GPLv2> I know my rights; I want my phone call!
<DRM> What use is a phone call, if you are unable to speak?
(as seen on /.)

2007-10-14  Robert Millan  <[email protected]>

	* loader/i386/pc/multiboot.c (grub_multiboot_load_elf32): When loading
	ELF segments, use a macro for arbitrarily accessing any of them instead
	of preparing a pointer that allows access to one at a time.
	(grub_multiboot_load_elf64): Likewise.

diff -ur grub2/loader/i386/pc/multiboot.c grub2.phdr_array/loader/i386/pc/multiboot.c

loader/i386/pc/multiboot.c +32 -26

@@ -94,7 +94,7 @@
 grub_multiboot_load_elf32 (grub_file_t file, void *buffer)
 {
   Elf32_Ehdr *ehdr = (Elf32_Ehdr *) buffer;
-  Elf32_Phdr *phdr;
+  void *phdr_base;
   int i;
 
   if (ehdr->e_ident[EI_CLASS] != ELFCLASS32)

@@ -112,35 +112,38 @@
 
   entry = ehdr->e_entry;
 
+  phdr_base = (void *) buffer + ehdr->e_phoff;
+#define phdr(i)		((Elf32_Phdr *) (phdr_base + (i) * ehdr->e_phentsize))
+
   /* Load every loadable segment in memory.  */
   for (i = 0; i < ehdr->e_phnum; i++)
     {
-      phdr = (Elf32_Phdr *) ((char *) buffer + ehdr->e_phoff
-			     + i * ehdr->e_phentsize);
-      if (phdr->p_type == PT_LOAD)
+      if (phdr(i)->p_type == PT_LOAD)
         {
           /* The segment should fit in the area reserved for the OS.  */
-          if ((phdr->p_paddr < grub_os_area_addr)
-              || (phdr->p_paddr + phdr->p_memsz
+          if ((phdr(i)->p_paddr < grub_os_area_addr)
+              || (phdr(i)->p_paddr + phdr(i)->p_memsz
 		  > grub_os_area_addr + grub_os_area_size))
 	    return grub_error (GRUB_ERR_BAD_OS,
 			       "segment doesn't fit in memory reserved for the OS");
 
-          if (grub_file_seek (file, (grub_off_t) phdr->p_offset)
+          if (grub_file_seek (file, (grub_off_t) phdr(i)->p_offset)
 	      == (grub_off_t) -1)
 	    return grub_error (GRUB_ERR_BAD_OS,
 			       "invalid offset in program header");
 
-          if (grub_file_read (file, (void *) phdr->p_paddr, phdr->p_filesz)
-              != (grub_ssize_t) phdr->p_filesz)
+          if (grub_file_read (file, (void *) phdr(i)->p_paddr, phdr(i)->p_filesz)
+              != (grub_ssize_t) phdr(i)->p_filesz)
 	    return grub_error (GRUB_ERR_BAD_OS,
 			       "couldn't read segment from file");
 
-          if (phdr->p_filesz < phdr->p_memsz)
-            grub_memset ((char *) phdr->p_paddr + phdr->p_filesz, 0,
-			 phdr->p_memsz - phdr->p_filesz);
+          if (phdr(i)->p_filesz < phdr(i)->p_memsz)
+            grub_memset ((char *) phdr(i)->p_paddr + phdr(i)->p_filesz, 0,
+			 phdr(i)->p_memsz - phdr(i)->p_filesz);
         }
     }
+
+#undef phdr
 
   return grub_errno;
 }

@@ -158,7 +161,7 @@
 grub_multiboot_load_elf64 (grub_file_t file, void *buffer)
 {
   Elf64_Ehdr *ehdr = (Elf64_Ehdr *) buffer;
-  Elf64_Phdr *phdr;
+  void *phdr_base;
   int i;
 
   if (ehdr->e_ident[EI_CLASS] != ELFCLASS64)

@@ -186,39 +189,42 @@
 
   entry = ehdr->e_entry;
 
+  phdr_base = (void *) buffer + ehdr->e_phoff;
+#define phdr(i)		((Elf64_Phdr *) (phdr_base + (i) * ehdr->e_phentsize))
+
   /* Load every loadable segment in memory.  */
   for (i = 0; i < ehdr->e_phnum; i++)
     {
-      phdr = (Elf64_Phdr *) ((char *) buffer + ehdr->e_phoff
-			     + i * ehdr->e_phentsize);
-      if (phdr->p_type == PT_LOAD)
+      if (phdr(i)->p_type == PT_LOAD)
         {
           /* The segment should fit in the area reserved for the OS.  */
-          if ((phdr->p_paddr < (grub_uint64_t) grub_os_area_addr)
-              || (phdr->p_paddr + phdr->p_memsz
+          if ((phdr(i)->p_paddr < (grub_uint64_t) grub_os_area_addr)
+              || (phdr(i)->p_paddr + phdr(i)->p_memsz
 		  > ((grub_uint64_t) grub_os_area_addr
 		     + (grub_uint64_t) grub_os_area_size)))
 	    return grub_error (GRUB_ERR_BAD_OS,
 			       "segment doesn't fit in memory reserved for the OS");
 
-	  if (grub_file_seek (file, (grub_off_t) phdr->p_offset)
+	  if (grub_file_seek (file, (grub_off_t) phdr(i)->p_offset)
 	      == (grub_off_t) -1)
 	    return grub_error (GRUB_ERR_BAD_OS,
 			       "invalid offset in program header");
 
-	  if (grub_file_read (file, (void *) ((grub_uint32_t) phdr->p_paddr),
-			      phdr->p_filesz)
-              != (grub_ssize_t) phdr->p_filesz)
+	  if (grub_file_read (file, (void *) ((grub_uint32_t) phdr(i)->p_paddr),
+			      phdr(i)->p_filesz)
+              != (grub_ssize_t) phdr(i)->p_filesz)
 	    return grub_error (GRUB_ERR_BAD_OS,
 			       "couldn't read segment from file");
 
-          if (phdr->p_filesz < phdr->p_memsz)
-	    grub_memset (((char *) ((grub_uint32_t) phdr->p_paddr)
-			  + phdr->p_filesz),
+          if (phdr(i)->p_filesz < phdr(i)->p_memsz)
+	    grub_memset (((char *) ((grub_uint32_t) phdr(i)->p_paddr)
+			  + phdr(i)->p_filesz),
 			 0,
-			 phdr->p_memsz - phdr->p_filesz);
+			 phdr(i)->p_memsz - phdr(i)->p_filesz);
         }
     }
+
+#undef phdr
 
   return grub_errno;
 }

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