dissect-image: add probe_sector_size() helper for detecting sector size of a GPT disk image

When we operate with DDIs with sector sizes != 512 we need to configure
the loopback device to match it, otherwise the image and the kernel
block device will disagree what things are.

Let's add a prober that tries to determine the sector size of a GPT DDI.
It does this by looking for the GPT partition table header at the
various byte offsets they must be located on, given a specific sector
size. It will try sector size 512, 1024, 2048 and 4096. Of these only
the 512 and 4096 really make sense IRL I guess, but let's be thorough.

src/shared/dissect-image.c

@@ -62,6 +62,7 @@
 #include "raw-clone.h"
 #include "resize-fs.h"
 #include "signal-util.h"
+#include "sparse-endian.h"
 #include "stat-util.h"
 #include "stdio-util.h"
 #include "string-table.h"
@@ -107,6 +108,87 @@ int dissect_fstype_ok(const char *fstype) {
         return false;
 }
 
+int probe_sector_size(int fd, uint32_t *ret) {
+
+        struct gpt_header {
+                char signature[8];
+                le32_t revision;
+                le32_t header_size;
+                le32_t crc32;
+                le32_t reserved;
+                le64_t my_lba;
+                le64_t alternate_lba;
+                le64_t first_usable_lba;
+                le64_t last_usable_lba;
+                sd_id128_t disk_guid;
+                le64_t partition_entry_lba;
+                le32_t number_of_partition_entries;
+                le32_t size_of_partition_entry;
+                le32_t partition_entry_array_crc32;
+        } _packed_;
+
+        /* Disk images might be for 512B or for 4096 sector sizes, let's try to auto-detect that by searching
+         * for the GPT headers at the relevant byte offsets */
+
+        assert_cc(sizeof(struct gpt_header) == 92);
+
+        /* We expect a sector size in the range 512…4096. The GPT header is located in the second
+         * sector. Hence it could be at byte 512 at the earliest, and at byte 4096 at the latest. And we must
+         * read with granularity of the largest sector size we care about. Which means 8K. */
+        uint8_t sectors[2 * 4096];
+        uint32_t found = 0;
+        ssize_t n;
+
+        assert(fd >= 0);
+        assert(ret);
+
+        n = pread(fd, sectors, sizeof(sectors), 0);
+        if (n < 0)
+                return -errno;
+        if (n != sizeof(sectors)) /* too short? */
+                goto not_found;
+
+        /* Let's see if we find the GPT partition header with various expected sector sizes */
+        for (uint32_t sz = 512; sz <= 4096; sz <<= 1) {
+                struct gpt_header *p;
+
+                assert(sizeof(sectors) >= sz * 2);
+                p = (struct gpt_header*) (sectors + sz);
+
+                if (memcmp(p->signature, (const char[8]) { 'E', 'F', 'I', ' ', 'P', 'A', 'R', 'T' }, 8) != 0)
+                        continue;
+
+                if (le32toh(p->revision) != UINT32_C(0x00010000)) /* the only known revision of the spec: 1.0 */
+                        continue;
+
+                if (le32toh(p->header_size) < sizeof(struct gpt_header))
+                        continue;
+
+                if (le32toh(p->header_size) > 4096) /* larger than a sector? something is off… */
+                        continue;
+
+                if (le64toh(p->my_lba) != 1) /* this sector must claim to be at sector offset 1 */
+                        continue;
+
+                if (found != 0)
+                        return log_debug_errno(SYNTHETIC_ERRNO(ENOTUNIQ),
+                                               "Detected valid partition table at offsets matching multiple sector sizes, refusing.");
+
+                found = sz;
+        }
+
+        if (found != 0) {
+                log_debug("Determined sector size %" PRIu32 " based on discovered partition table.", found);
+                *ret = found;
+                return 1; /* indicate we *did* find it */
+        }
+
+not_found:
+        log_debug("Couldn't find any partition table to derive sector size of.");
+        *ret = 512; /* pick the traditional default */
+        return 0;   /* indicate we didn't find it */
+}
+
 int probe_filesystem_full(
                 int fd,
                 const char *path,

src/shared/dissect-image.h

@@ -186,3 +186,5 @@ int mount_image_privately_interactively(const char *path, DissectImageFlags flag
 int verity_dissect_and_mount(int src_fd, const char *src, const char *dest, const MountOptions *options, const char *required_host_os_release_id, const char *required_host_os_release_version_id, const char *required_host_os_release_sysext_level, const char *required_sysext_scope);
 
 int dissect_fstype_ok(const char *fstype);
+
+int probe_sector_size(int fd, uint32_t *ret);