Re: [PATCH v4 10/20] rust: add `kernel` crate

[Petr Mladek <pmladek@suse.com>]

On Sat 2022-02-12 14:03:36, Miguel Ojeda wrote:
> From: Wedson Almeida Filho <wedsonaf@google.com>
> 
> The `kernel` crate currently includes all the abstractions that wrap
> kernel features written in C.
> 
> These abstractions call the C side of the kernel via the generated
> bindings with the `bindgen` tool. Modules developed in Rust should
> never call the bindings themselves.
> 
> In the future, as the abstractions grow in number, we may need
> to split this crate into several, possibly following a similar
> subdivision in subsystems as the kernel itself and/or moving
> the code to the actual subsystems.
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 82abfaf3c2aa..c042386667f2 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -392,7 +392,10 @@ static struct latched_seq clear_seq = {
>  /* the maximum size of a formatted record (i.e. with prefix added per line) */
>  #define CONSOLE_LOG_MAX		1024
>  
> -/* the maximum size allowed to be reserved for a record */
> +/*
> + * The maximum size allowed to be reserved for a record.
> + * Keep in sync with rust/kernel/print.rs.
> + */
>  #define LOG_LINE_MAX		(CONSOLE_LOG_MAX - PREFIX_MAX)

What exactly should we keep in sync, please?

I see only handling of KERN_* prefix in print.rs. I do not see there
any counter part of LOG_LINE_MAX, CONSOLE_LOG_MAX, or PREFIX_MAX.

Also note that PREFIX_MAX is the maximal lenght of the prefix printed
on console. It is log level + time stamp + caller id. For example:

<12>[  123.632345][ T3260]

It seems that print.rs defines max size of the prefix in the printk
format where the log level is defined using KERN_* pair of characters.

>  
>  #define LOG_LEVEL(v)		((v) & 0x07)
> diff --git a/rust/kernel/print.rs b/rust/kernel/print.rs
> new file mode 100644
> index 000000000000..dba4ef10c722
> --- /dev/null
> +++ b/rust/kernel/print.rs
> @@ -0,0 +1,417 @@
> +/// Format strings.
> +///
> +/// Public but hidden since it should only be used from public macros.
> +#[doc(hidden)]
> +pub mod format_strings {
> +    use crate::bindings;
> +
> +    /// The length we copy from the `KERN_*` kernel prefixes.
> +    const LENGTH_PREFIX: usize = 2;
> +
> +    /// The length of the fixed format strings.
> +    pub const LENGTH: usize = 10;

I am sorry but I am not familiar with rust. What are these limits
2 and 10 used for, please?

I guess that 2 is the size of a single KERN_* identifier.
But what is 10?

Note that printk() format prefix is typically just a single KERN_*
identifier. But there might be more. Well, in practice only the
following combination makes sense: KERN_CONT + KERN_<LEVEL>.


> +    /// Generates a fixed format string for the kernel's [`_printk`].
> +    ///
> +    /// The format string is always the same for a given level, i.e. for a
> +    /// given `prefix`, which are the kernel's `KERN_*` constants.
> +    ///
> +    /// [`_printk`]: ../../../../include/linux/printk.h
> +    const fn generate(is_cont: bool, prefix: &[u8; 3]) -> [u8; LENGTH] {
> +        // Ensure the `KERN_*` macros are what we expect.
> +        assert!(prefix[0] == b'\x01');
> +        if is_cont {
> +            assert!(prefix[1] == b'c');
> +        } else {
> +            assert!(prefix[1] >= b'0' && prefix[1] <= b'7');
> +        }
> +        assert!(prefix[2] == b'\x00');
> +
> +        let suffix: &[u8; LENGTH - LENGTH_PREFIX] = if is_cont {
> +            b"%pA\0\0\0\0\0"
> +        } else {
> +            b"%s: %pA\0"
> +        };
> +
> +        [
> +            prefix[0], prefix[1], suffix[0], suffix[1], suffix[2], suffix[3], suffix[4], suffix[5],
> +            suffix[6], suffix[7],
> +        ]
> +    }

Finally, is there any way to test whether any change in the printk
code breaks the rust support?

Best Regards,
Petr