Re: [PATCH] ACPI: Implement overriding of arbitrary ACPI tables via initrd

H. Peter Anvin
2012-03-26T01:37:42Z

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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.

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

[-- Attachment #2: findcpio.c --]
[-- Type: text/x-csrc, Size: 2505 bytes --]

/*
* 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;
}