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

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Date:	Mon, 26 Mar 2012 10:21:35 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Borislav Petkov <bp@...en8.de>, Thomas Renninger <trenn@...e.de>,
	eric.piel@...mplin-utc.net, vojcek@...n.pl, dsdt@...gusch.at,
	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
	x86@...nel.org, Lin Ming <ming.m.lin@...el.com>, lenb@...nel.org,
	robert.moore@...el.com, Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] ACPI: Implement overriding of arbitrary ACPI tables via
 initrd

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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