On Sun, Mar 25, 2012 at 06:36:56PM -0700, H. Peter Anvin wrote:
> On 03/25/2012 01:54 AM, Borislav Petkov wrote:
> >
> > One other downside for this approach would be the need to have
> > initrd/initramfs support always compiled into the kernel in order to do
> > all those things but, hey, distro kernels already have that so we're
> > probably stuck with it anyway.
> >
>
> In theory we could make the early stuff independent of the initramfs
> code in the kernel -- just like we always build in an initramfs.
Were would this payload resize? Would it be tacked on the initramfs.gz
or perhaps to the vmlinuz? Or would it be a seperate payload (so multiboot
type)? Or would this be supported by all of those mechanism?
>
> Not that it matters much, at this point the no-initramfs configurations
> are mostly theoretical.
>
> I was able to shave a bit more off the code... optimizing for size can
> be fun sometimes. The code is now 442 bytes on x86-64 and 413 on i386
> when compiled with -O2 -fomit-frame-pointer; with -Os
> -fomit-frame-pointer it is 372/410... all of that without any library
> calls or relocations of any kind (which means it should be safe to call
> from the prepaging environment.)
>
> -hpa
> /*
> * findcpio.c
> *
> * Find a specific cpio member; must precede any compressed content.
> */
>
> #include <stddef.h>
> #include <stdbool.h>
>
> struct cpio_data {
> void *data;
> unsigned long size;
> };
>
> enum cpio_fields {
> C_MAGIC,
> C_INO,
> C_MODE,
> C_UID,
> C_GID,
> C_NLINK,
> C_MTIME,
> C_FILESIZE,
> C_MAJ,
> C_MIN,
> C_RMAJ,
> C_RMIN,
> C_NAMESIZE,
> C_CHKSUM,
> C_NFIELDS
> };
>
> #if defined(__i386__) || defined(__x86_64__)
> static size_t strlen(const char *name)
> {
> size_t n = -1;
>
> asm("repne; scasb"
> : "+D" (name), "+c" (n)
> : "a" (0));
>
> return -2 - n;
> }
>
> static int memcmp(const void *p1, const void *p2, size_t n)
> {
> unsigned char rv;
>
> asm("repe; cmpsb; setne %0"
> : "=r" (rv), "+S" (p1), "+D" (p2), "+c" (n));
>
> return rv;
> }
> #else
> static size_t strlen(const char *name)
> {
> size_t n = 0;
>
> while (*name++)
> n++;
>
> return n;
> }
>
> static int memcmp(const void *p1, const void *p2, size_t n)
> {
> const unsigned char *u1 = p1;
> const unsigned char *u2 = p2;
> int d;
>
> while (n--) {
> d = *u2++ - *u1++;
> if (d)
> return d;
> }
> return 0;
> }
> #endif
>
> #define ALIGN4(p) ((void *)(((size_t)p + 3) & ~3))
>
> struct cpio_data find_cpio_data(const char *name, const void *data, size_t len)
> {
> const size_t cpio_header_len = 8*C_NFIELDS - 2;
> struct cpio_data cd = { NULL, 0 };
> const char *p, *dptr, *nptr;
> unsigned int ch[C_NFIELDS], *chp, v;
> unsigned char c, x;
> size_t mynamesize = strlen(name) + 1;
> int i, j;
>
> p = data;
>
> while (len > cpio_header_len) {
> if (!*p) {
> /* All cpio headers need to be 4-byte aligned */
> p += 4;
> len -= 4;
> continue;
> }
>
> j = 6; /* The magic field is only 6 characters */
> chp = ch;
> for (i = C_NFIELDS; i; i--) {
> v = 0;
> while (j--) {
> v <<= 4;
> c = *p++;
>
> x = c - '0';
> if (x < 10) {
> v += x;
> continue;
> }
>
> x = (c | 0x20) - 'a';
> if (x < 6) {
> v += x + 10;
> continue;
> }
>
> goto quit; /* Invalid hexadecimal */
> }
> *chp++ = v;
> j = 8; /* All other fields are 8 characters */
> }
>
> if ((ch[C_MAGIC] - 0x070701) > 1)
> goto quit; /* Invalid magic */
>
> len -= cpio_header_len;
>
> dptr = ALIGN4(p + ch[C_NAMESIZE]);
> nptr = ALIGN4(dptr + ch[C_FILESIZE]);
>
> if (nptr > p + len || dptr < p || nptr < dptr)
> goto quit; /* Buffer overrun */
>
> if ((ch[C_MODE] & 0170000) == 0100000 &&
> ch[C_NAMESIZE] == mynamesize &&
> !memcmp(p, name, mynamesize)) {
> cd.data = (void *)dptr;
> cd.size = ch[C_FILESIZE];
> return cd; /* Found it! */
> }
>
> len -= (nptr - p);
> p = nptr;
> }
>
> quit:
> return cd;
> }
>
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