blob: 23d38fb484be22c90f94d72f89045d38fef14f77 [file] [log] [blame] [raw]
/* SPDX-License-Identifier: LGPL-2.1+ */
#include <dirent.h>
#include <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <linux/fs.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <unistd.h>
#include "sd-id128.h"
#include "alloc-util.h"
#include "chattr-util.h"
#include "dirent-util.h"
#include "efivars.h"
#include "fd-util.h"
#include "io-util.h"
#include "macro.h"
#include "parse-util.h"
#include "stdio-util.h"
#include "strv.h"
#include "time-util.h"
#include "utf8.h"
#include "util.h"
#include "virt.h"
#if ENABLE_EFI
#define LOAD_OPTION_ACTIVE 0x00000001
#define MEDIA_DEVICE_PATH 0x04
#define MEDIA_HARDDRIVE_DP 0x01
#define MEDIA_FILEPATH_DP 0x04
#define SIGNATURE_TYPE_GUID 0x02
#define MBR_TYPE_EFI_PARTITION_TABLE_HEADER 0x02
#define END_DEVICE_PATH_TYPE 0x7f
#define END_ENTIRE_DEVICE_PATH_SUBTYPE 0xff
#define EFI_OS_INDICATIONS_BOOT_TO_FW_UI 0x0000000000000001
struct boot_option {
uint32_t attr;
uint16_t path_len;
uint16_t title[];
} _packed_;
struct drive_path {
uint32_t part_nr;
uint64_t part_start;
uint64_t part_size;
char signature[16];
uint8_t mbr_type;
uint8_t signature_type;
} _packed_;
struct device_path {
uint8_t type;
uint8_t sub_type;
uint16_t length;
union {
uint16_t path[0];
struct drive_path drive;
};
} _packed_;
bool is_efi_boot(void) {
if (detect_container() > 0)
return false;
return access("/sys/firmware/efi/", F_OK) >= 0;
}
static int read_flag(const char *varname) {
_cleanup_free_ void *v = NULL;
uint8_t b;
size_t s;
int r;
if (!is_efi_boot()) /* If this is not an EFI boot, assume the queried flags are zero */
return 0;
r = efi_get_variable(EFI_VENDOR_GLOBAL, varname, NULL, &v, &s);
if (r < 0)
return r;
if (s != 1)
return -EINVAL;
b = *(uint8_t *)v;
return !!b;
}
bool is_efi_secure_boot(void) {
return read_flag("SecureBoot") > 0;
}
bool is_efi_secure_boot_setup_mode(void) {
return read_flag("SetupMode") > 0;
}
int efi_reboot_to_firmware_supported(void) {
_cleanup_free_ void *v = NULL;
uint64_t b;
size_t s;
int r;
if (!is_efi_boot())
return -EOPNOTSUPP;
r = efi_get_variable(EFI_VENDOR_GLOBAL, "OsIndicationsSupported", NULL, &v, &s);
if (r == -ENOENT) /* variable doesn't exist? it's not supported then */
return -EOPNOTSUPP;
if (r < 0)
return r;
if (s != sizeof(uint64_t))
return -EINVAL;
b = *(uint64_t*) v;
if (!(b & EFI_OS_INDICATIONS_BOOT_TO_FW_UI))
return -EOPNOTSUPP; /* bit unset? it's not supported then */
return 0;
}
static int get_os_indications(uint64_t *os_indication) {
_cleanup_free_ void *v = NULL;
size_t s;
int r;
/* Let's verify general support first */
r = efi_reboot_to_firmware_supported();
if (r < 0)
return r;
r = efi_get_variable(EFI_VENDOR_GLOBAL, "OsIndications", NULL, &v, &s);
if (r == -ENOENT) {
/* Some firmware implementations that do support OsIndications and report that with
* OsIndicationsSupported will remove the OsIndications variable when it is unset. Let's pretend it's 0
* then, to hide this implementation detail. Note that this call will return -ENOENT then only if the
* support for OsIndications is missing entirely, as determined by efi_reboot_to_firmware_supported()
* above. */
*os_indication = 0;
return 0;
} else if (r < 0)
return r;
else if (s != sizeof(uint64_t))
return -EINVAL;
*os_indication = *(uint64_t *)v;
return 0;
}
int efi_get_reboot_to_firmware(void) {
int r;
uint64_t b;
r = get_os_indications(&b);
if (r < 0)
return r;
return !!(b & EFI_OS_INDICATIONS_BOOT_TO_FW_UI);
}
int efi_set_reboot_to_firmware(bool value) {
int r;
uint64_t b, b_new;
r = get_os_indications(&b);
if (r < 0)
return r;
if (value)
b_new = b | EFI_OS_INDICATIONS_BOOT_TO_FW_UI;
else
b_new = b & ~EFI_OS_INDICATIONS_BOOT_TO_FW_UI;
/* Avoid writing to efi vars store if we can due to firmware bugs. */
if (b != b_new)
return efi_set_variable(EFI_VENDOR_GLOBAL, "OsIndications", &b_new, sizeof(uint64_t));
return 0;
}
int efi_get_variable(
sd_id128_t vendor,
const char *name,
uint32_t *attribute,
void **value,
size_t *size) {
_cleanup_close_ int fd = -1;
_cleanup_free_ char *p = NULL;
uint32_t a;
ssize_t n;
struct stat st;
_cleanup_free_ void *buf = NULL;
assert(name);
assert(value);
assert(size);
if (asprintf(&p,
"/sys/firmware/efi/efivars/%s-%02x%02x%02x%02x-%02x%02x-%02x%02x-%02x%02x-%02x%02x%02x%02x%02x%02x",
name, SD_ID128_FORMAT_VAL(vendor)) < 0)
return -ENOMEM;
fd = open(p, O_RDONLY|O_NOCTTY|O_CLOEXEC);
if (fd < 0)
return -errno;
if (fstat(fd, &st) < 0)
return -errno;
if (st.st_size < 4)
return -EIO;
if (st.st_size > 4*1024*1024 + 4)
return -E2BIG;
n = read(fd, &a, sizeof(a));
if (n < 0)
return -errno;
if (n != sizeof(a))
return -EIO;
buf = malloc(st.st_size - 4 + 2);
if (!buf)
return -ENOMEM;
n = read(fd, buf, (size_t) st.st_size - 4);
if (n < 0)
return -errno;
if (n != (ssize_t) st.st_size - 4)
return -EIO;
/* Always NUL terminate (2 bytes, to protect UTF-16) */
((char*) buf)[st.st_size - 4] = 0;
((char*) buf)[st.st_size - 4 + 1] = 0;
*value = TAKE_PTR(buf);
*size = (size_t) st.st_size - 4;
if (attribute)
*attribute = a;
return 0;
}
int efi_get_variable_string(sd_id128_t vendor, const char *name, char **p) {
_cleanup_free_ void *s = NULL;
size_t ss = 0;
int r;
char *x;
r = efi_get_variable(vendor, name, NULL, &s, &ss);
if (r < 0)
return r;
x = utf16_to_utf8(s, ss);
if (!x)
return -ENOMEM;
*p = x;
return 0;
}
int efi_set_variable(
sd_id128_t vendor,
const char *name,
const void *value,
size_t size) {
struct var {
uint32_t attr;
char buf[];
} _packed_ * _cleanup_free_ buf = NULL;
_cleanup_free_ char *p = NULL;
_cleanup_close_ int fd = -1;
bool saved_flags_valid = false;
unsigned saved_flags;
int r;
assert(name);
assert(value || size == 0);
if (asprintf(&p,
"/sys/firmware/efi/efivars/%s-%02x%02x%02x%02x-%02x%02x-%02x%02x-%02x%02x-%02x%02x%02x%02x%02x%02x",
name, SD_ID128_FORMAT_VAL(vendor)) < 0)
return -ENOMEM;
/* Newer efivarfs protects variables that are not in a whitelist with FS_IMMUTABLE_FL by default, to protect
* them for accidental removal and modification. We are not changing these variables accidentally however,
* hence let's unset the bit first. */
r = chattr_path(p, 0, FS_IMMUTABLE_FL, &saved_flags);
if (r < 0 && r != -ENOENT)
log_debug_errno(r, "Failed to drop FS_IMMUTABLE_FL flag from '%s', ignoring: %m", p);
saved_flags_valid = r >= 0;
if (size == 0) {
if (unlink(p) < 0) {
r = -errno;
goto finish;
}
return 0;
}
fd = open(p, O_WRONLY|O_CREAT|O_NOCTTY|O_CLOEXEC, 0644);
if (fd < 0) {
r = -errno;
goto finish;
}
buf = malloc(sizeof(uint32_t) + size);
if (!buf) {
r = -ENOMEM;
goto finish;
}
buf->attr = EFI_VARIABLE_NON_VOLATILE|EFI_VARIABLE_BOOTSERVICE_ACCESS|EFI_VARIABLE_RUNTIME_ACCESS;
memcpy(buf->buf, value, size);
r = loop_write(fd, buf, sizeof(uint32_t) + size, false);
if (r < 0)
goto finish;
r = 0;
finish:
if (saved_flags_valid) {
int q;
/* Restore the original flags field, just in case */
if (fd < 0)
q = chattr_path(p, saved_flags, FS_IMMUTABLE_FL, NULL);
else
q = chattr_fd(fd, saved_flags, FS_IMMUTABLE_FL, NULL);
if (q < 0)
log_debug_errno(q, "Failed to restore FS_IMMUTABLE_FL on '%s', ignoring: %m", p);
}
return r;
}
int efi_set_variable_string(sd_id128_t vendor, const char *name, const char *v) {
_cleanup_free_ char16_t *u16 = NULL;
u16 = utf8_to_utf16(v, strlen(v));
if (!u16)
return -ENOMEM;
return efi_set_variable(vendor, name, u16, (char16_strlen(u16) + 1) * sizeof(char16_t));
}
static size_t utf16_size(const uint16_t *s) {
size_t l = 0;
while (s[l] > 0)
l++;
return (l+1) * sizeof(uint16_t);
}
static void efi_guid_to_id128(const void *guid, sd_id128_t *id128) {
struct uuid {
uint32_t u1;
uint16_t u2;
uint16_t u3;
uint8_t u4[8];
} _packed_;
const struct uuid *uuid = guid;
id128->bytes[0] = (uuid->u1 >> 24) & 0xff;
id128->bytes[1] = (uuid->u1 >> 16) & 0xff;
id128->bytes[2] = (uuid->u1 >> 8) & 0xff;
id128->bytes[3] = (uuid->u1) & 0xff;
id128->bytes[4] = (uuid->u2 >> 8) & 0xff;
id128->bytes[5] = (uuid->u2) & 0xff;
id128->bytes[6] = (uuid->u3 >> 8) & 0xff;
id128->bytes[7] = (uuid->u3) & 0xff;
memcpy(&id128->bytes[8], uuid->u4, sizeof(uuid->u4));
}
int efi_get_boot_option(
uint16_t id,
char **title,
sd_id128_t *part_uuid,
char **path,
bool *active) {
char boot_id[9];
_cleanup_free_ uint8_t *buf = NULL;
size_t l;
struct boot_option *header;
size_t title_size;
_cleanup_free_ char *s = NULL, *p = NULL;
sd_id128_t p_uuid = SD_ID128_NULL;
int r;
if (!is_efi_boot())
return -EOPNOTSUPP;
xsprintf(boot_id, "Boot%04X", id);
r = efi_get_variable(EFI_VENDOR_GLOBAL, boot_id, NULL, (void **)&buf, &l);
if (r < 0)
return r;
if (l < sizeof(struct boot_option))
return -ENOENT;
header = (struct boot_option *)buf;
title_size = utf16_size(header->title);
if (title_size > l - offsetof(struct boot_option, title))
return -EINVAL;
if (title) {
s = utf16_to_utf8(header->title, title_size);
if (!s)
return -ENOMEM;
}
if (header->path_len > 0) {
uint8_t *dbuf;
size_t dnext, doff;
doff = offsetof(struct boot_option, title) + title_size;
dbuf = buf + doff;
if (header->path_len > l - doff)
return -EINVAL;
dnext = 0;
while (dnext < header->path_len) {
struct device_path *dpath;
dpath = (struct device_path *)(dbuf + dnext);
if (dpath->length < 4)
break;
/* Type 0x7F – End of Hardware Device Path, Sub-Type 0xFF – End Entire Device Path */
if (dpath->type == END_DEVICE_PATH_TYPE && dpath->sub_type == END_ENTIRE_DEVICE_PATH_SUBTYPE)
break;
dnext += dpath->length;
/* Type 0x04 – Media Device Path */
if (dpath->type != MEDIA_DEVICE_PATH)
continue;
/* Sub-Type 1 – Hard Drive */
if (dpath->sub_type == MEDIA_HARDDRIVE_DP) {
/* 0x02 – GUID Partition Table */
if (dpath->drive.mbr_type != MBR_TYPE_EFI_PARTITION_TABLE_HEADER)
continue;
/* 0x02 – GUID signature */
if (dpath->drive.signature_type != SIGNATURE_TYPE_GUID)
continue;
if (part_uuid)
efi_guid_to_id128(dpath->drive.signature, &p_uuid);
continue;
}
/* Sub-Type 4 – File Path */
if (dpath->sub_type == MEDIA_FILEPATH_DP && !p && path) {
p = utf16_to_utf8(dpath->path, dpath->length-4);
if (!p)
return -ENOMEM;
efi_tilt_backslashes(p);
continue;
}
}
}
if (title)
*title = TAKE_PTR(s);
if (part_uuid)
*part_uuid = p_uuid;
if (path)
*path = TAKE_PTR(p);
if (active)
*active = !!(header->attr & LOAD_OPTION_ACTIVE);
return 0;
}
static void to_utf16(uint16_t *dest, const char *src) {
int i;
for (i = 0; src[i] != '\0'; i++)
dest[i] = src[i];
dest[i] = '\0';
}
struct guid {
uint32_t u1;
uint16_t u2;
uint16_t u3;
uint8_t u4[8];
} _packed_;
static void id128_to_efi_guid(sd_id128_t id, void *guid) {
struct guid *uuid = guid;
uuid->u1 = id.bytes[0] << 24 | id.bytes[1] << 16 | id.bytes[2] << 8 | id.bytes[3];
uuid->u2 = id.bytes[4] << 8 | id.bytes[5];
uuid->u3 = id.bytes[6] << 8 | id.bytes[7];
memcpy(uuid->u4, id.bytes+8, sizeof(uuid->u4));
}
static uint16_t *tilt_slashes(uint16_t *s) {
uint16_t *p;
for (p = s; *p; p++)
if (*p == '/')
*p = '\\';
return s;
}
int efi_add_boot_option(
uint16_t id,
const char *title,
uint32_t part,
uint64_t pstart,
uint64_t psize,
sd_id128_t part_uuid,
const char *path) {
size_t size, title_len, path_len;
_cleanup_free_ char *buf = NULL;
struct boot_option *option;
struct device_path *devicep;
char boot_id[9];
if (!is_efi_boot())
return -EOPNOTSUPP;
title_len = (strlen(title)+1) * 2;
path_len = (strlen(path)+1) * 2;
buf = malloc0(sizeof(struct boot_option) + title_len +
sizeof(struct drive_path) +
sizeof(struct device_path) + path_len);
if (!buf)
return -ENOMEM;
/* header */
option = (struct boot_option *)buf;
option->attr = LOAD_OPTION_ACTIVE;
option->path_len = offsetof(struct device_path, drive) + sizeof(struct drive_path) +
offsetof(struct device_path, path) + path_len +
offsetof(struct device_path, path);
to_utf16(option->title, title);
size = offsetof(struct boot_option, title) + title_len;
/* partition info */
devicep = (struct device_path *)(buf + size);
devicep->type = MEDIA_DEVICE_PATH;
devicep->sub_type = MEDIA_HARDDRIVE_DP;
devicep->length = offsetof(struct device_path, drive) + sizeof(struct drive_path);
devicep->drive.part_nr = part;
devicep->drive.part_start = pstart;
devicep->drive.part_size = psize;
devicep->drive.signature_type = SIGNATURE_TYPE_GUID;
devicep->drive.mbr_type = MBR_TYPE_EFI_PARTITION_TABLE_HEADER;
id128_to_efi_guid(part_uuid, devicep->drive.signature);
size += devicep->length;
/* path to loader */
devicep = (struct device_path *)(buf + size);
devicep->type = MEDIA_DEVICE_PATH;
devicep->sub_type = MEDIA_FILEPATH_DP;
devicep->length = offsetof(struct device_path, path) + path_len;
to_utf16(devicep->path, path);
tilt_slashes(devicep->path);
size += devicep->length;
/* end of path */
devicep = (struct device_path *)(buf + size);
devicep->type = END_DEVICE_PATH_TYPE;
devicep->sub_type = END_ENTIRE_DEVICE_PATH_SUBTYPE;
devicep->length = offsetof(struct device_path, path);
size += devicep->length;
xsprintf(boot_id, "Boot%04X", id);
return efi_set_variable(EFI_VENDOR_GLOBAL, boot_id, buf, size);
}
int efi_remove_boot_option(uint16_t id) {
char boot_id[9];
if (!is_efi_boot())
return -EOPNOTSUPP;
xsprintf(boot_id, "Boot%04X", id);
return efi_set_variable(EFI_VENDOR_GLOBAL, boot_id, NULL, 0);
}
int efi_get_boot_order(uint16_t **order) {
_cleanup_free_ void *buf = NULL;
size_t l;
int r;
if (!is_efi_boot())
return -EOPNOTSUPP;
r = efi_get_variable(EFI_VENDOR_GLOBAL, "BootOrder", NULL, &buf, &l);
if (r < 0)
return r;
if (l <= 0)
return -ENOENT;
if (l % sizeof(uint16_t) > 0 ||
l / sizeof(uint16_t) > INT_MAX)
return -EINVAL;
*order = TAKE_PTR(buf);
return (int) (l / sizeof(uint16_t));
}
int efi_set_boot_order(uint16_t *order, size_t n) {
if (!is_efi_boot())
return -EOPNOTSUPP;
return efi_set_variable(EFI_VENDOR_GLOBAL, "BootOrder", order, n * sizeof(uint16_t));
}
static int boot_id_hex(const char s[static 4]) {
int id = 0, i;
for (i = 0; i < 4; i++)
if (s[i] >= '0' && s[i] <= '9')
id |= (s[i] - '0') << (3 - i) * 4;
else if (s[i] >= 'A' && s[i] <= 'F')
id |= (s[i] - 'A' + 10) << (3 - i) * 4;
else
return -EINVAL;
return id;
}
static int cmp_uint16(const uint16_t *a, const uint16_t *b) {
return CMP(*a, *b);
}
int efi_get_boot_options(uint16_t **options) {
_cleanup_closedir_ DIR *dir = NULL;
_cleanup_free_ uint16_t *list = NULL;
struct dirent *de;
size_t alloc = 0;
int count = 0;
assert(options);
if (!is_efi_boot())
return -EOPNOTSUPP;
dir = opendir("/sys/firmware/efi/efivars/");
if (!dir)
return -errno;
FOREACH_DIRENT(de, dir, return -errno) {
int id;
if (strncmp(de->d_name, "Boot", 4) != 0)
continue;
if (strlen(de->d_name) != 45)
continue;
if (strcmp(de->d_name + 8, "-8be4df61-93ca-11d2-aa0d-00e098032b8c") != 0)
continue;
id = boot_id_hex(de->d_name + 4);
if (id < 0)
continue;
if (!GREEDY_REALLOC(list, alloc, count + 1))
return -ENOMEM;
list[count++] = id;
}
typesafe_qsort(list, count, cmp_uint16);
*options = TAKE_PTR(list);
return count;
}
static int read_usec(sd_id128_t vendor, const char *name, usec_t *u) {
_cleanup_free_ char *j = NULL;
int r;
uint64_t x = 0;
assert(name);
assert(u);
r = efi_get_variable_string(EFI_VENDOR_LOADER, name, &j);
if (r < 0)
return r;
r = safe_atou64(j, &x);
if (r < 0)
return r;
*u = x;
return 0;
}
int efi_loader_get_boot_usec(usec_t *firmware, usec_t *loader) {
uint64_t x, y;
int r;
assert(firmware);
assert(loader);
if (!is_efi_boot())
return -EOPNOTSUPP;
r = read_usec(EFI_VENDOR_LOADER, "LoaderTimeInitUSec", &x);
if (r < 0)
return r;
r = read_usec(EFI_VENDOR_LOADER, "LoaderTimeExecUSec", &y);
if (r < 0)
return r;
if (y == 0 || y < x)
return -EIO;
if (y > USEC_PER_HOUR)
return -EIO;
*firmware = x;
*loader = y;
return 0;
}
int efi_loader_get_device_part_uuid(sd_id128_t *u) {
_cleanup_free_ char *p = NULL;
int r, parsed[16];
if (!is_efi_boot())
return -EOPNOTSUPP;
r = efi_get_variable_string(EFI_VENDOR_LOADER, "LoaderDevicePartUUID", &p);
if (r < 0)
return r;
if (sscanf(p, "%02x%02x%02x%02x-%02x%02x-%02x%02x-%02x%02x-%02x%02x%02x%02x%02x%02x",
&parsed[0], &parsed[1], &parsed[2], &parsed[3],
&parsed[4], &parsed[5], &parsed[6], &parsed[7],
&parsed[8], &parsed[9], &parsed[10], &parsed[11],
&parsed[12], &parsed[13], &parsed[14], &parsed[15]) != 16)
return -EIO;
if (u) {
unsigned i;
for (i = 0; i < ELEMENTSOF(parsed); i++)
u->bytes[i] = parsed[i];
}
return 0;
}
bool efi_loader_entry_name_valid(const char *s) {
if (isempty(s))
return false;
if (strlen(s) > FILENAME_MAX) /* Make sure entry names fit in filenames */
return false;
return in_charset(s, ALPHANUMERICAL "-");
}
int efi_loader_get_entries(char ***ret) {
_cleanup_free_ char16_t *entries = NULL;
_cleanup_strv_free_ char **l = NULL;
size_t size, i, start;
int r;
assert(ret);
if (!is_efi_boot())
return -EOPNOTSUPP;
r = efi_get_variable(EFI_VENDOR_LOADER, "LoaderEntries", NULL, (void**) &entries, &size);
if (r < 0)
return r;
/* The variable contains a series of individually NUL terminated UTF-16 strings. */
for (i = 0, start = 0;; i++) {
_cleanup_free_ char *decoded = NULL;
bool end;
/* Is this the end of the variable's data? */
end = i * sizeof(char16_t) >= size;
/* Are we in the middle of a string? (i.e. not at the end of the variable, nor at a NUL terminator?) If
* so, let's go to the next entry. */
if (!end && entries[i] != 0)
continue;
/* We reached the end of a string, let's decode it into UTF-8 */
decoded = utf16_to_utf8(entries + start, (i - start) * sizeof(char16_t));
if (!decoded)
return -ENOMEM;
if (efi_loader_entry_name_valid(decoded)) {
r = strv_consume(&l, TAKE_PTR(decoded));
if (r < 0)
return r;
} else
log_debug("Ignoring invalid loader entry '%s'.", decoded);
/* We reached the end of the variable */
if (end)
break;
/* Continue after the NUL byte */
start = i + 1;
}
*ret = TAKE_PTR(l);
return 0;
}
int efi_loader_get_features(uint64_t *ret) {
_cleanup_free_ void *v = NULL;
size_t s;
int r;
if (!is_efi_boot()) {
*ret = 0;
return 0;
}
r = efi_get_variable(EFI_VENDOR_LOADER, "LoaderFeatures", NULL, &v, &s);
if (r == -ENOENT) {
_cleanup_free_ char *info = NULL;
/* The new (v240+) LoaderFeatures variable is not supported, let's see if it's systemd-boot at all */
r = efi_get_variable_string(EFI_VENDOR_LOADER, "LoaderInfo", &info);
if (r < 0) {
if (r != -ENOENT)
return r;
/* Variable not set, definitely means not systemd-boot */
} else if (first_word(info, "systemd-boot")) {
/* An older systemd-boot version. Let's hardcode the feature set, since it was pretty
* static in all its versions. */
*ret = EFI_LOADER_FEATURE_CONFIG_TIMEOUT |
EFI_LOADER_FEATURE_ENTRY_DEFAULT |
EFI_LOADER_FEATURE_ENTRY_ONESHOT;
return 0;
}
/* No features supported */
*ret = 0;
return 0;
}
if (r < 0)
return r;
if (s != sizeof(uint64_t))
return log_debug_errno(SYNTHETIC_ERRNO(EINVAL),
"LoaderFeatures EFI variable doesn't have the right size.");
memcpy(ret, v, sizeof(uint64_t));
return 0;
}
#endif
char *efi_tilt_backslashes(char *s) {
char *p;
for (p = s; *p; p++)
if (*p == '\\')
*p = '/';
return s;
}