[RFC] kexec: Add bootconfig support to get_command_line

[Paul E. McKenney]

On Mon, Aug 11, 2025 at 04:37:36PM +0100, Usama Arif wrote:
> If the kernel is enabled with CONFIG_BOOT_CONFIG_FORCE, it will
> always append the bootconfig at the start of the kernel commandline.
> If reuse-cmdline option is used in kexec, the bootconfig will be
> repeatedly appended at the start of the commandline on every kexec.
> As there is a limit on the size of kernel commandline that can be
> used, --reuse-cmdline option will break kexec depending on the
> size of bootconfig on repeated kexec.
> 
> Bootconfig is embedded in the kernel, so it should not be considered
> when reusing kernel command line.
> Modify get_command_line() to first attempt reading kernel parameters
> from /proc/bootconfig before falling back to /proc/cmdline. This allows
> kexec to use the original boot parameters without bootconfig which
> is embedded in the kernel.
> 
> The function will fallback to /proc/cmdline behavior when:
> 
> - /proc/bootconfig doesn't exist
> - /proc/bootconfig is empty
> - /proc/bootconfig doesn't contain the expected marker format
> - The extracted parameter line is empty
> 
> Signed-off-by: Usama Arif <usamaarif642 at gmail.com>

Reviewed-by: Paul E. McKenney <paulmck at kernel.org>

> ---
>  kexec/kexec.c | 50 ++++++++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 44 insertions(+), 6 deletions(-)
> 
> diff --git a/kexec/kexec.c b/kexec/kexec.c
> index 6bf12d7..b31cb1f 100644
> --- a/kexec/kexec.c
> +++ b/kexec/kexec.c
> @@ -1235,15 +1235,53 @@ static char *slurp_proc_file(const char *filename, size_t *len)
>   */
>  char *get_command_line(void)
>  {
> -	char *p, *line;
> +	char *p, *line = NULL;
>  	size_t size;
> +	char *bootconfig_line;
> +	size_t bootconfig_size;
> +
> +	/* First try to get command line parameters from /proc/bootconfig */
> +	bootconfig_line = slurp_proc_file("/proc/bootconfig", &bootconfig_size);
> +	if (bootconfig_line && bootconfig_size > 0) {
> +		/* Look for "# Parameters from bootloader:" */
> +		char *params_marker = strstr(bootconfig_line, "# Parameters from bootloader:");
> +		if (params_marker) {
> +			/* Find the next line after the marker */
> +			char *params_start = strchr(params_marker, '\n');
> +			if (params_start) {
> +				params_start++; /* Move past the newline */
> +
> +				/* Check if this line starts with "# " */
> +				if (strncmp(params_start, "# ", 2) == 0) {
> +					/* Skip the "# " prefix */
> +					params_start += 2;
> +
> +					/* Find the end of the line */
> +					char *params_end = strchr(params_start, '\n');
> +					if (params_end) {
> +						*params_end = '\0';
> +					}
> +
> +					/* Check if the extracted line is not empty */
> +					if (strlen(params_start) > 0) {
> +						/* Allocate and copy the parameters */
> +						line = xstrdup(params_start);
> +					}
> +				}
> +			}
> +		}
> +		free(bootconfig_line);
> +	}
>  
> -	line = slurp_proc_file("/proc/cmdline", &size);
> -	if (!line || !size)
> -		die("Failed to read /proc/cmdline\n");
> +	/* Fall back to reading /proc/cmdline if we didn't get a line from bootconfig */
> +	if (!line) {
> +		line = slurp_proc_file("/proc/cmdline", &size);
> +		if (!line || !size)
> +			die("Failed to read /proc/cmdline\n");
>  
> -	/* strip newline */
> -	line[size-1] = '\0';
> +		/* strip newline */
> +		line[size-1] = '\0';
> +	}
>  
>  	p = strpbrk(line, "\r\n");
>  	if (p)
> -- 
> 2.47.3
>