x86/kprobes: Fix crash when probing CS CALL instructionsgrafted

When using eBPF to probe CS CALL instructions within a function,
a crash can be triggered.

The eBPF tool probes offset 257 of the __hrtimer_run_queues()
function:

<__hrtimer_run_queues+249>:  nopl   0x0(%rax,%rax,1)
<__hrtimer_run_queues+254>:  mov    %r14,%rdi
<__hrtimer_run_queues+257>:  cs call <__x86_indirect_thunk_r12>
<__hrtimer_run_queues+263>:  mov    %eax,%r12d
<__hrtimer_run_queues+266>:  xchg   %ax,%ax
<__hrtimer_run_queues+268>:  mov    %r13,%rdi

Which triggers this crash:

  BUG: unable to handle page fault for address: 00000000000f41c9
  #PF: supervisor write access in kernel mode
  #PF: error_code(0x0002) - not-present page
  PGD 0 P4D 0
  Oops: 0002 [#1] SMP NOPTI
  CPU: 1 PID: 0 Comm: swapper/1 Kdump: loaded Tainted: P
  RIP: 0010:__hrtimer_run_queues+0x106/0x230

Note that __hrtimer_run_queues+0x106 is __hrtimer_run_queues+262, which is
at the 6th byte of the above CS CALL instruction. Since the CS CALL
instruction occupies 6 bytes, the exception occurred in the middle of that
call instruction.

The root cause is that when using eBPF tools to probe in the middle of a
function, a kprobe with INT3 is used as the underlying implementation.

During single-step emulation of the original CALL instruction,
int3_emulate_call() assumes that the probed CALL instruction is 5 bytes
long. However, the actual CS-prefixed CALL instruction occupies 6 bytes,
so it constructs an incorrect exception return address. When the CPU
returns from the kprobe handler, the next instruction to be executed is at
the address of the last byte of that CS CALL instruction. Coincidentally,
starting from that address, the CPU fetches and decodes a completely
different instruction, which ultimately triggers a kernel crash.

Fix the issue by using the actual instruction length obtained from
the instruction decoder when constructing the exception return
address, rather than relying on the hardcoded CALL_INSN_SIZE macro.

[ mingo: Refined the changelog ]

Fixes: 6256e668b7af ("x86/kprobes: Use int3 instead of debug trap for single-step")
Suggested-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Jinke Han <jinkehan@didiglobal.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://patch.msgid.link/20260908073742.GA10517@didi-ThinkCentre-M920t-N000

1 files changed, 600 insertions, 0 deletions

diff --git a/drivers/scsi/sr_ioctl.c b/drivers/scsi/sr_ioctl.c
new file mode 100644
index 000000000..089653018
--- /dev/null
+++ b/drivers/scsi/sr_ioctl.c
@@ -0,0 +1,600 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/kernel.h>
+#include <linux/mm.h>
+#include <linux/fs.h>
+#include <linux/errno.h>
+#include <linux/string.h>
+#include <linux/blkdev.h>
+#include <linux/module.h>
+#include <linux/blkpg.h>
+#include <linux/cdrom.h>
+#include <linux/delay.h>
+#include <linux/slab.h>
+#include <asm/io.h>
+#include <linux/uaccess.h>
+
+#include <scsi/scsi.h>
+#include <scsi/scsi_dbg.h>
+#include <scsi/scsi_device.h>
+#include <scsi/scsi_eh.h>
+#include <scsi/scsi_host.h>
+#include <scsi/scsi_ioctl.h>
+#include <scsi/scsi_cmnd.h>
+
+#include "sr.h"
+
+#if 0
+#define DEBUG
+#endif
+
+/* The sr_is_xa() seems to trigger firmware bugs with some drives :-(
+ * It is off by default and can be turned on with this module parameter */
+static int xa_test = 0;
+
+module_param(xa_test, int, S_IRUGO | S_IWUSR);
+
+static int sr_read_tochdr(struct cdrom_device_info *cdi,
+		struct cdrom_tochdr *tochdr)
+{
+	struct scsi_cd *cd = cdi->handle;
+	struct packet_command cgc;
+	int result;
+	unsigned char *buffer;
+
+	buffer = kzalloc(32, GFP_KERNEL);
+	if (!buffer)
+		return -ENOMEM;
+
+	memset(&cgc, 0, sizeof(struct packet_command));
+	cgc.timeout = IOCTL_TIMEOUT;
+	cgc.cmd[0] = GPCMD_READ_TOC_PMA_ATIP;
+	cgc.cmd[8] = 12;		/* LSB of length */
+	cgc.buffer = buffer;
+	cgc.buflen = 12;
+	cgc.quiet = 1;
+	cgc.data_direction = DMA_FROM_DEVICE;
+
+	result = sr_do_ioctl(cd, &cgc);
+	if (result)
+		goto err;
+
+	tochdr->cdth_trk0 = buffer[2];
+	tochdr->cdth_trk1 = buffer[3];
+
+err:
+	kfree(buffer);
+	return result;
+}
+
+static int sr_read_tocentry(struct cdrom_device_info *cdi,
+		struct cdrom_tocentry *tocentry)
+{
+	struct scsi_cd *cd = cdi->handle;
+	struct packet_command cgc;
+	int result;
+	unsigned char *buffer;
+
+	buffer = kzalloc(32, GFP_KERNEL);
+	if (!buffer)
+		return -ENOMEM;
+
+	memset(&cgc, 0, sizeof(struct packet_command));
+	cgc.timeout = IOCTL_TIMEOUT;
+	cgc.cmd[0] = GPCMD_READ_TOC_PMA_ATIP;
+	cgc.cmd[1] |= (tocentry->cdte_format == CDROM_MSF) ? 0x02 : 0;
+	cgc.cmd[6] = tocentry->cdte_track;
+	cgc.cmd[8] = 12;		/* LSB of length */
+	cgc.buffer = buffer;
+	cgc.buflen = 12;
+	cgc.data_direction = DMA_FROM_DEVICE;
+
+	result = sr_do_ioctl(cd, &cgc);
+	if (result)
+		goto err;
+
+	tocentry->cdte_ctrl = buffer[5] & 0xf;
+	tocentry->cdte_adr = buffer[5] >> 4;
+	tocentry->cdte_datamode = (tocentry->cdte_ctrl & 0x04) ? 1 : 0;
+	if (tocentry->cdte_format == CDROM_MSF) {
+		tocentry->cdte_addr.msf.minute = buffer[9];
+		tocentry->cdte_addr.msf.second = buffer[10];
+		tocentry->cdte_addr.msf.frame = buffer[11];
+	} else
+		tocentry->cdte_addr.lba = (((((buffer[8] << 8) + buffer[9]) << 8)
+			+ buffer[10]) << 8) + buffer[11];
+
+err:
+	kfree(buffer);
+	return result;
+}
+
+#define IOCTL_RETRIES 3
+
+/* ATAPI drives don't have a SCMD_PLAYAUDIO_TI command.  When these drives
+   are emulating a SCSI device via the idescsi module, they need to have
+   CDROMPLAYTRKIND commands translated into CDROMPLAYMSF commands for them */
+
+static int sr_fake_playtrkind(struct cdrom_device_info *cdi, struct cdrom_ti *ti)
+{
+	struct cdrom_tocentry trk0_te, trk1_te;
+	struct cdrom_tochdr tochdr;
+	struct packet_command cgc;
+	int ntracks, ret;
+
+	ret = sr_read_tochdr(cdi, &tochdr);
+	if (ret)
+		return ret;
+
+	ntracks = tochdr.cdth_trk1 - tochdr.cdth_trk0 + 1;
+	
+	if (ti->cdti_trk1 == ntracks) 
+		ti->cdti_trk1 = CDROM_LEADOUT;
+	else if (ti->cdti_trk1 != CDROM_LEADOUT)
+		ti->cdti_trk1 ++;
+
+	trk0_te.cdte_track = ti->cdti_trk0;
+	trk0_te.cdte_format = CDROM_MSF;
+	trk1_te.cdte_track = ti->cdti_trk1;
+	trk1_te.cdte_format = CDROM_MSF;
+	
+	ret = sr_read_tocentry(cdi, &trk0_te);
+	if (ret)
+		return ret;
+	ret = sr_read_tocentry(cdi, &trk1_te);
+	if (ret)
+		return ret;
+
+	memset(&cgc, 0, sizeof(struct packet_command));
+	cgc.cmd[0] = GPCMD_PLAY_AUDIO_MSF;
+	cgc.cmd[3] = trk0_te.cdte_addr.msf.minute;
+	cgc.cmd[4] = trk0_te.cdte_addr.msf.second;
+	cgc.cmd[5] = trk0_te.cdte_addr.msf.frame;
+	cgc.cmd[6] = trk1_te.cdte_addr.msf.minute;
+	cgc.cmd[7] = trk1_te.cdte_addr.msf.second;
+	cgc.cmd[8] = trk1_te.cdte_addr.msf.frame;
+	cgc.data_direction = DMA_NONE;
+	cgc.timeout = IOCTL_TIMEOUT;
+	return sr_do_ioctl(cdi->handle, &cgc);
+}
+
+static int sr_play_trkind(struct cdrom_device_info *cdi,
+		struct cdrom_ti *ti)
+
+{
+	struct scsi_cd *cd = cdi->handle;
+	struct packet_command cgc;
+	int result;
+
+	memset(&cgc, 0, sizeof(struct packet_command));
+	cgc.timeout = IOCTL_TIMEOUT;
+	cgc.cmd[0] = GPCMD_PLAYAUDIO_TI;
+	cgc.cmd[4] = ti->cdti_trk0;
+	cgc.cmd[5] = ti->cdti_ind0;
+	cgc.cmd[7] = ti->cdti_trk1;
+	cgc.cmd[8] = ti->cdti_ind1;
+	cgc.data_direction = DMA_NONE;
+
+	result = sr_do_ioctl(cd, &cgc);
+	if (result == -EDRIVE_CANT_DO_THIS)
+		result = sr_fake_playtrkind(cdi, ti);
+
+	return result;
+}
+
+/* We do our own retries because we want to know what the specific
+   error code is.  Normally the UNIT_ATTENTION code will automatically
+   clear after one error */
+
+int sr_do_ioctl(Scsi_CD *cd, struct packet_command *cgc)
+{
+	struct scsi_device *SDev;
+	struct scsi_sense_hdr local_sshdr, *sshdr;
+	int result, err = 0, retries = 0;
+	const struct scsi_exec_args exec_args = {
+		.sshdr = cgc->sshdr ? : &local_sshdr,
+	};
+
+	SDev = cd->device;
+
+	sshdr = exec_args.sshdr;
+
+      retry:
+	if (!scsi_block_when_processing_errors(SDev)) {
+		err = -ENODEV;
+		goto out;
+	}
+
+	result = scsi_execute_cmd(SDev, cgc->cmd,
+				  cgc->data_direction == DMA_TO_DEVICE ?
+				  REQ_OP_DRV_OUT : REQ_OP_DRV_IN, cgc->buffer,
+				  cgc->buflen, cgc->timeout, IOCTL_RETRIES,
+				  &exec_args);
+	/* Minimal error checking.  Ignore cases we know about, and report the rest. */
+	if (result < 0) {
+		err = result;
+		goto out;
+	}
+	if (scsi_status_is_check_condition(result)) {
+		switch (sshdr->sense_key) {
+		case UNIT_ATTENTION:
+			SDev->changed = 1;
+			if (!cgc->quiet)
+				sr_printk(KERN_INFO, cd,
+					  "disc change detected.\n");
+			if (retries++ < 10)
+				goto retry;
+			err = -ENOMEDIUM;
+			break;
+		case NOT_READY:	/* This happens if there is no disc in drive */
+			if (sshdr->asc == 0x04 &&
+			    sshdr->ascq == 0x01) {
+				/* sense: Logical unit is in process of becoming ready */
+				if (!cgc->quiet)
+					sr_printk(KERN_INFO, cd,
+						  "CDROM not ready yet.\n");
+				if (retries++ < 10) {
+					/* sleep 2 sec and try again */
+					ssleep(2);
+					goto retry;
+				} else {
+					/* 20 secs are enough? */
+					err = -ENOMEDIUM;
+					break;
+				}
+			}
+			if (!cgc->quiet)
+				sr_printk(KERN_INFO, cd,
+					  "CDROM not ready.  Make sure there "
+					  "is a disc in the drive.\n");
+			err = -ENOMEDIUM;
+			break;
+		case ILLEGAL_REQUEST:
+			err = -EIO;
+			if (sshdr->asc == 0x20 &&
+			    sshdr->ascq == 0x00)
+				/* sense: Invalid command operation code */
+				err = -EDRIVE_CANT_DO_THIS;
+			break;
+		default:
+			err = -EIO;
+		}
+	}
+
+	/* Wake up a process waiting for device */
+      out:
+	cgc->stat = err;
+	return err;
+}
+
+/* ---------------------------------------------------------------------- */
+/* interface to cdrom.c                                                   */
+
+int sr_tray_move(struct cdrom_device_info *cdi, int pos)
+{
+	Scsi_CD *cd = cdi->handle;
+	struct packet_command cgc;
+
+	memset(&cgc, 0, sizeof(struct packet_command));
+	cgc.cmd[0] = GPCMD_START_STOP_UNIT;
+	cgc.cmd[4] = (pos == 0) ? 0x03 /* close */ : 0x02 /* eject */ ;
+	cgc.data_direction = DMA_NONE;
+	cgc.timeout = IOCTL_TIMEOUT;
+	return sr_do_ioctl(cd, &cgc);
+}
+
+int sr_lock_door(struct cdrom_device_info *cdi, int lock)
+{
+	Scsi_CD *cd = cdi->handle;
+
+	return scsi_set_medium_removal(cd->device, lock ?
+		       SCSI_REMOVAL_PREVENT : SCSI_REMOVAL_ALLOW);
+}
+
+int sr_drive_status(struct cdrom_device_info *cdi, int slot)
+{
+	struct scsi_cd *cd = cdi->handle;
+	struct scsi_sense_hdr sshdr;
+	struct media_event_desc med;
+
+	if (CDSL_CURRENT != slot) {
+		/* we have no changer support */
+		return -EINVAL;
+	}
+	if (!scsi_test_unit_ready(cd->device, SR_TIMEOUT, MAX_RETRIES, &sshdr))
+		return CDS_DISC_OK;
+
+	/* SK/ASC/ASCQ of 2/4/1 means "unit is becoming ready" */
+	if (scsi_sense_valid(&sshdr) && sshdr.sense_key == NOT_READY
+			&& sshdr.asc == 0x04 && sshdr.ascq == 0x01)
+		return CDS_DRIVE_NOT_READY;
+
+	if (!cdrom_get_media_event(cdi, &med)) {
+		if (med.media_present)
+			return CDS_DISC_OK;
+		else if (med.door_open)
+			return CDS_TRAY_OPEN;
+		else
+			return CDS_NO_DISC;
+	}
+
+	/*
+	 * SK/ASC/ASCQ of 2/4/2 means "initialization required"
+	 * Using CD_TRAY_OPEN results in an START_STOP_UNIT to close
+	 * the tray, which resolves the initialization requirement.
+	 */
+	if (scsi_sense_valid(&sshdr) && sshdr.sense_key == NOT_READY
+			&& sshdr.asc == 0x04 && sshdr.ascq == 0x02)
+		return CDS_TRAY_OPEN;
+
+	/*
+	 * 0x04 is format in progress .. but there must be a disc present!
+	 */
+	if (sshdr.sense_key == NOT_READY && sshdr.asc == 0x04)
+		return CDS_DISC_OK;
+
+	/*
+	 * If not using Mt Fuji extended media tray reports,
+	 * just return TRAY_OPEN since ATAPI doesn't provide
+	 * any other way to detect this...
+	 */
+	if (scsi_sense_valid(&sshdr) &&
+	    /* 0x3a is medium not present */
+	    sshdr.asc == 0x3a)
+		return CDS_NO_DISC;
+	else
+		return CDS_TRAY_OPEN;
+
+	return CDS_DRIVE_NOT_READY;
+}
+
+int sr_disk_status(struct cdrom_device_info *cdi)
+{
+	Scsi_CD *cd = cdi->handle;
+	struct cdrom_tochdr toc_h;
+	struct cdrom_tocentry toc_e;
+	int i, rc, have_datatracks = 0;
+
+	/* look for data tracks */
+	rc = sr_read_tochdr(cdi, &toc_h);
+	if (rc)
+		return (rc == -ENOMEDIUM) ? CDS_NO_DISC : CDS_NO_INFO;
+
+	for (i = toc_h.cdth_trk0; i <= toc_h.cdth_trk1; i++) {
+		toc_e.cdte_track = i;
+		toc_e.cdte_format = CDROM_LBA;
+		if (sr_read_tocentry(cdi, &toc_e))
+			return CDS_NO_INFO;
+		if (toc_e.cdte_ctrl & CDROM_DATA_TRACK) {
+			have_datatracks = 1;
+			break;
+		}
+	}
+	if (!have_datatracks)
+		return CDS_AUDIO;
+
+	if (cd->xa_flag)
+		return CDS_XA_2_1;
+	else
+		return CDS_DATA_1;
+}
+
+int sr_get_last_session(struct cdrom_device_info *cdi,
+			struct cdrom_multisession *ms_info)
+{
+	Scsi_CD *cd = cdi->handle;
+
+	ms_info->addr.lba = cd->ms_offset;
+	ms_info->xa_flag = cd->xa_flag || cd->ms_offset > 0;
+
+	return 0;
+}
+
+int sr_get_mcn(struct cdrom_device_info *cdi, struct cdrom_mcn *mcn)
+{
+	Scsi_CD *cd = cdi->handle;
+	struct packet_command cgc;
+	char *buffer = kzalloc(32, GFP_KERNEL);
+	int result;
+
+	if (!buffer)
+		return -ENOMEM;
+
+	memset(&cgc, 0, sizeof(struct packet_command));
+	cgc.cmd[0] = GPCMD_READ_SUBCHANNEL;
+	cgc.cmd[2] = 0x40;	/* I do want the subchannel info */
+	cgc.cmd[3] = 0x02;	/* Give me medium catalog number info */
+	cgc.cmd[8] = 24;
+	cgc.buffer = buffer;
+	cgc.buflen = 24;
+	cgc.data_direction = DMA_FROM_DEVICE;
+	cgc.timeout = IOCTL_TIMEOUT;
+	result = sr_do_ioctl(cd, &cgc);
+	if (result)
+		goto err;
+
+	memcpy(mcn->medium_catalog_number, buffer + 9, 13);
+	mcn->medium_catalog_number[13] = 0;
+
+err:
+	kfree(buffer);
+	return result;
+}
+
+int sr_reset(struct cdrom_device_info *cdi)
+{
+	return 0;
+}
+
+int sr_select_speed(struct cdrom_device_info *cdi, unsigned long speed)
+{
+	Scsi_CD *cd = cdi->handle;
+	struct packet_command cgc;
+
+	/* avoid exceeding the max speed or overflowing integer bounds */
+	speed = clamp(speed, 0, 0xffff / 177);
+
+	if (speed == 0)
+		speed = 0xffff;	/* set to max */
+	else
+		speed *= 177;	/* Nx to kbyte/s */
+
+	memset(&cgc, 0, sizeof(struct packet_command));
+	cgc.cmd[0] = GPCMD_SET_SPEED;	/* SET CD SPEED */
+	cgc.cmd[2] = (speed >> 8) & 0xff;	/* MSB for speed (in kbytes/sec) */
+	cgc.cmd[3] = speed & 0xff;	/* LSB */
+	cgc.data_direction = DMA_NONE;
+	cgc.timeout = IOCTL_TIMEOUT;
+
+	if (sr_do_ioctl(cd, &cgc))
+		return -EIO;
+	return 0;
+}
+
+/* ----------------------------------------------------------------------- */
+/* this is called by the generic cdrom driver. arg is a _kernel_ pointer,  */
+/* because the generic cdrom driver does the user access stuff for us.     */
+/* only cdromreadtochdr and cdromreadtocentry are left - for use with the  */
+/* sr_disk_status interface for the generic cdrom driver.                  */
+
+int sr_audio_ioctl(struct cdrom_device_info *cdi, unsigned int cmd, void *arg)
+{
+	switch (cmd) {
+	case CDROMREADTOCHDR:
+		return sr_read_tochdr(cdi, arg);
+	case CDROMREADTOCENTRY:
+		return sr_read_tocentry(cdi, arg);
+	case CDROMPLAYTRKIND:
+		return sr_play_trkind(cdi, arg);
+	default:
+		return -EINVAL;
+	}
+}
+
+/* -----------------------------------------------------------------------
+ * a function to read all sorts of funny cdrom sectors using the READ_CD
+ * scsi-3 mmc command
+ *
+ * lba:     linear block address
+ * format:  0 = data (anything)
+ *          1 = audio
+ *          2 = data (mode 1)
+ *          3 = data (mode 2)
+ *          4 = data (mode 2 form1)
+ *          5 = data (mode 2 form2)
+ * blksize: 2048 | 2336 | 2340 | 2352
+ */
+
+static int sr_read_cd(Scsi_CD *cd, unsigned char *dest, int lba, int format, int blksize)
+{
+	struct packet_command cgc;
+
+#ifdef DEBUG
+	sr_printk(KERN_INFO, cd, "sr_read_cd lba=%d format=%d blksize=%d\n",
+		  lba, format, blksize);
+#endif
+
+	memset(&cgc, 0, sizeof(struct packet_command));
+	cgc.cmd[0] = GPCMD_READ_CD;	/* READ_CD */
+	cgc.cmd[1] = ((format & 7) << 2);
+	cgc.cmd[2] = (unsigned char) (lba >> 24) & 0xff;
+	cgc.cmd[3] = (unsigned char) (lba >> 16) & 0xff;
+	cgc.cmd[4] = (unsigned char) (lba >> 8) & 0xff;
+	cgc.cmd[5] = (unsigned char) lba & 0xff;
+	cgc.cmd[8] = 1;
+	switch (blksize) {
+	case 2336:
+		cgc.cmd[9] = 0x58;
+		break;
+	case 2340:
+		cgc.cmd[9] = 0x78;
+		break;
+	case 2352:
+		cgc.cmd[9] = 0xf8;
+		break;
+	default:
+		cgc.cmd[9] = 0x10;
+		break;
+	}
+	cgc.buffer = dest;
+	cgc.buflen = blksize;
+	cgc.data_direction = DMA_FROM_DEVICE;
+	cgc.timeout = IOCTL_TIMEOUT;
+	return sr_do_ioctl(cd, &cgc);
+}
+
+/*
+ * read sectors with blocksizes other than 2048
+ */
+
+static int sr_read_sector(Scsi_CD *cd, int lba, int blksize, unsigned char *dest)
+{
+	struct packet_command cgc;
+	int rc;
+
+	/* we try the READ CD command first... */
+	if (cd->readcd_known) {
+		rc = sr_read_cd(cd, dest, lba, 0, blksize);
+		if (-EDRIVE_CANT_DO_THIS != rc)
+			return rc;
+		cd->readcd_known = 0;
+		sr_printk(KERN_INFO, cd,
+			  "CDROM doesn't support READ CD (0xbe) command\n");
+		/* fall & retry the other way */
+	}
+	/* ... if this fails, we switch the blocksize using MODE SELECT */
+	if (blksize != cd->device->sector_size) {
+		if (0 != (rc = sr_set_blocklength(cd, blksize)))
+			return rc;
+	}
+#ifdef DEBUG
+	sr_printk(KERN_INFO, cd, "sr_read_sector lba=%d blksize=%d\n",
+		  lba, blksize);
+#endif
+
+	memset(&cgc, 0, sizeof(struct packet_command));
+	cgc.cmd[0] = GPCMD_READ_10;
+	cgc.cmd[2] = (unsigned char) (lba >> 24) & 0xff;
+	cgc.cmd[3] = (unsigned char) (lba >> 16) & 0xff;
+	cgc.cmd[4] = (unsigned char) (lba >> 8) & 0xff;
+	cgc.cmd[5] = (unsigned char) lba & 0xff;
+	cgc.cmd[8] = 1;
+	cgc.buffer = dest;
+	cgc.buflen = blksize;
+	cgc.data_direction = DMA_FROM_DEVICE;
+	cgc.timeout = IOCTL_TIMEOUT;
+	rc = sr_do_ioctl(cd, &cgc);
+
+	if (blksize != CD_FRAMESIZE)
+		rc |= sr_set_blocklength(cd, CD_FRAMESIZE);
+	return rc;
+}
+
+/*
+ * read a sector in raw mode to check the sector format
+ * ret: 1 == mode2 (XA), 0 == mode1, <0 == error 
+ */
+
+int sr_is_xa(Scsi_CD *cd)
+{
+	unsigned char *raw_sector;
+	int is_xa;
+
+	if (!xa_test)
+		return 0;
+
+	raw_sector = kmalloc(2048, GFP_KERNEL);
+	if (!raw_sector)
+		return -ENOMEM;
+	if (0 == sr_read_sector(cd, cd->ms_offset + 16,
+				CD_FRAMESIZE_RAW1, raw_sector)) {
+		is_xa = (raw_sector[3] == 0x02) ? 1 : 0;
+	} else {
+		/* read a raw sector failed for some reason. */
+		is_xa = -1;
+	}
+	kfree(raw_sector);
+#ifdef DEBUG
+	sr_printk(KERN_INFO, cd, "sr_is_xa: %d\n", is_xa);
+#endif
+	return is_xa;
+}