doc: document charset to use for bootspec entry names

Prompted-by: https://github.com/systemd/systemd/issues/12572#issuecomment-711074702

diff --git a/docs/BOOT_LOADER_SPECIFICATION.md b/docs/BOOT_LOADER_SPECIFICATION.md
index 803ba5440faffc6a3a76120fa9222dcd5e161505..715b14e0e493ba524c1cd04b0567ae05f2a79a31 100644 (file)
--- a/docs/BOOT_LOADER_SPECIFICATION.md
+++ b/docs/BOOT_LOADER_SPECIFICATION.md
@@ -114,7 +114,23 @@ We define two directories below `$BOOT`:
 
 **Note:** _In all cases the `/loader/` directory should be located directly in the root of the file system. Specifically, if `$BOOT` is the ESP, then `/loader/` directory should be located directly in the root directory of the ESP, and not in the `/EFI/` subdirectory._
 
-Inside the `$BOOT/loader/entries/` directory each OS vendor may drop one or more configuration snippets with the suffix ".conf", one for each boot menu item. The file name of the file is used for identification of the boot item but shall never be presented to the user in the UI. The file name may be chosen freely but should be unique enough to avoid clashes between OS installations. More specifically it is suggested to include the machine ID (`/etc/machine-id` or the D-Bus machine ID for OSes that lack `/etc/machine-id`), the kernel version (as returned by `uname -r`) and an OS identifier (The ID field of `/etc/os-release`). Example: `$BOOT/loader/entries/6a9857a393724b7a981ebb5b8495b9ea-3.8.0-2.fc19.x86_64.conf`.
+Inside the `$BOOT/loader/entries/` directory each OS vendor may drop one or
+more configuration snippets with the suffix ".conf", one for each boot menu
+item. The file name of the file is used for identification of the boot item but
+shall never be presented to the user in the UI. The file name may be chosen
+freely but should be unique enough to avoid clashes between OS
+installations. More specifically it is suggested to include the machine ID
+(`/etc/machine-id` or the D-Bus machine ID for OSes that lack
+`/etc/machine-id`), the kernel version (as returned by `uname -r`) and an OS
+identifier (The ID field of `/etc/os-release`). Example:
+`$BOOT/loader/entries/6a9857a393724b7a981ebb5b8495b9ea-3.8.0-2.fc19.x86_64.conf`.
+
+In order to maximize compatibility with file system implementations and
+restricted boot loader environments, and to minimize conflicting character use
+with other progams, file names shall be chosen from a restricted character set:
+ASCII upper and lower case characters, digits, "+", "-", "_" and ".". Also, the
+file names should have a length of at least one and at most 255 characters
+(including file name suffix).
 
 These configuration snippets shall be Unix-style text files (i.e. line separation with a single newline character), in the UTF-8 encoding. The configuration snippets are loosely inspired on Grub1's configuration syntax. Lines beginning with '#' shall be ignored and used for commenting. The first word of a line is used as key and shall be separated by one or more spaces from its value. The following keys are known:
 
@@ -171,6 +187,10 @@ extension `.efi`. Support for images of this type is of course specific to
 systems with EFI firmware. Ignore this section if you work on systems not
 supporting EFI.
 
+Type #2 file names should be chosen from the same restricted character set as
+Type #1 described above (but use a different file name suffix of `.efi` instead
+of `.conf`).
+
 Images of this type have the advantage that all metadata and payload that makes
 up the boot entry is monopolized in a single PE file that can be signed
 cryptographically as one for the purpose of EFI SecureBoot.