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SubjectRe: [PATCH] ACPI: Implement overriding of arbitrary ACPI tables via initrd
On 03/24/2012 03:21 PM, H. Peter Anvin wrote:
> The attached cpio-parsing code compiles to 458 bytes on x86-64 and 476
> bytes on i386, and that is without any library dependencies at all.
> Again, it will completely stop at the first compressed data item, so any
> such kernel objects absolutely will have to be first. In good Linux
> tradition, it is also completely untested.
>
> However, given that very reasonable size I would think that this is a
> reasonable approach. Anyone who has a better suggestion for the
> namespace than "kernel/"?
>

Slightly improved version with actually working memcmp()...

-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_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
};

/* Return true if this field is composed of valid hex digits */
static bool validhex(const char *ptr, int len)
{
unsigned char c, x;

while (len--) {
c = *ptr++;

x = c - '0';
if (x < 10)
continue;

x = (c | 0x20) - 'a' + 10;
if (x < 16)
continue;

return false;
}

return true;
}

/* Return the value of an already validated field */
static unsigned int hexval(const char *ptr, int len)
{
unsigned int v = 0;
unsigned char c, x;

while (len--) {
v <<= 4;
c = *ptr++;

x = c - '0';
if (x < 10) {
v += x;
continue;
}

x = (c | 0x20) - 'a' + 10;
v += x;
}

return v;
}

#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 = 6 + 8*C_NFIELDS;
struct cpio_data cd = { NULL, 0 };
const char *p, *dptr, *nptr;
unsigned int magic, ch[C_NFIELDS], *chp;
size_t mynamesize = strlen(name) + 1;
int i;

p = data;

while (len > cpio_header_len) {
if (!*p) {
/* All cpio headers need to be 4-byte aligned */
p += 4;
len -= 4;
continue;
}

if (!validhex(p, cpio_header_len))
break; /* Not a valid cpio header */

magic = hexval(p, 6);
if ((magic - 0x070701) > 1)
break; /* Not a valid cpio magic */

p += 6;
chp = ch;
for (i = 0; i < C_NFIELDS; i++) {
*chp++ = hexval(p, 8);
p += 8;

}

len -= cpio_header_len;

dptr = ALIGN4(p + ch[C_NAMESIZE]);
nptr = ALIGN4(dptr + ch[C_FILESIZE]);

if (nptr > p + len)
break; /* 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];
break;
}

len -= (nptr - p);
p = nptr;
}

return cd;
}

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