Re: [PATCH 5/7] x86: kexec_file: lift CRASH_MAX_RANGES limit on crash_mem buffer

[Dave Young]

On 02/27/18 at 01:48pm, AKASHI Takahiro wrote:
> While CRASH_MAX_RANGES (== 16) seems to be good enough, fixed-number
> array is not a good idea in general.
> 
> In this patch, size of crash_mem buffer is calculated as before and
> the buffer is now dynamically allocated. This change also allows removing
> crash_elf_data structure.
> 
> Signed-off-by: AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx>
> Cc: Dave Young <dyoung@xxxxxxxxxx>
> Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
> Cc: Baoquan He <bhe@xxxxxxxxxx>
> ---
>  arch/x86/kernel/crash.c | 80 ++++++++++++++++++-------------------------------
>  1 file changed, 29 insertions(+), 51 deletions(-)
> 
> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
> index 913fd8021f8a..bfc37ad20d4a 100644
> --- a/arch/x86/kernel/crash.c
> +++ b/arch/x86/kernel/crash.c
> @@ -41,32 +41,14 @@
>  /* Alignment required for elf header segment */
>  #define ELF_CORE_HEADER_ALIGN   4096
>  
> -/* This primarily represents number of split ranges due to exclusion */
> -#define CRASH_MAX_RANGES	16
> -
>  struct crash_mem_range {
>  	u64 start, end;
>  };
>  
>  struct crash_mem {
> -	unsigned int nr_ranges;
> -	struct crash_mem_range ranges[CRASH_MAX_RANGES];
> -};
> -
> -/* Misc data about ram ranges needed to prepare elf headers */
> -struct crash_elf_data {
> -	struct kimage *image;
> -	/*
> -	 * Total number of ram ranges we have after various adjustments for
> -	 * crash reserved region, etc.
> -	 */
>  	unsigned int max_nr_ranges;
> -
> -	/* Pointer to elf header */
> -	void *ehdr;
> -	/* Pointer to next phdr */
> -	void *bufp;
> -	struct crash_mem mem;
> +	unsigned int nr_ranges;
> +	struct crash_mem_range ranges[0];
>  };
>  
>  /* Used while preparing memory map entries for second kernel */
> @@ -217,26 +199,29 @@ static int get_nr_ram_ranges_callback(struct resource *res, void *arg)
>  	return 0;
>  }
>  
> -
>  /* Gather all the required information to prepare elf headers for ram regions */
> -static void fill_up_crash_elf_data(struct crash_elf_data *ced,
> -				   struct kimage *image)
> +static struct crash_mem *fill_up_crash_elf_data(void)
>  {
>  	unsigned int nr_ranges = 0;
> -
> -	ced->image = image;
> +	struct crash_mem *cmem;
>  
>  	walk_system_ram_res(0, -1, &nr_ranges,
>  				get_nr_ram_ranges_callback);

I know it is probably not possible fail here, but for safe we can check
if nr_ranges == 0

>  
> -	ced->max_nr_ranges = nr_ranges;
> +	/*
> +	 * Exclusion of crash region and/or crashk_low_res may cause
> +	 * another range split. So add extra two slots here.
> +	 */
> +	nr_ranges += 2;
> +	cmem = vmalloc(sizeof(struct crash_mem) +
> +			sizeof(struct crash_mem_range) * nr_ranges);
> +	if (!cmem)
> +		return NULL;

vzalloc will be better. 

>  
> -	/* Exclusion of crash region could split memory ranges */
> -	ced->max_nr_ranges++;
> +	cmem->max_nr_ranges = nr_ranges;
> +	cmem->nr_ranges = 0;
>  
> -	/* If crashk_low_res is not 0, another range split possible */
> -	if (crashk_low_res.end)
> -		ced->max_nr_ranges++;
> +	return cmem;
>  }
>  
>  static int exclude_mem_range(struct crash_mem *mem,
> @@ -293,10 +278,8 @@ static int exclude_mem_range(struct crash_mem *mem,
>  		return 0;
>  
>  	/* Split happened */
> -	if (i == CRASH_MAX_RANGES - 1) {
> -		pr_err("Too many crash ranges after split\n");
> +	if (i == mem->max_nr_ranges - 1)
>  		return -ENOMEM;
> -	}
>  
>  	/* Location where new range should go */
>  	j = i + 1;
> @@ -314,11 +297,10 @@ static int exclude_mem_range(struct crash_mem *mem,
>  
>  /*
>   * Look for any unwanted ranges between mstart, mend and remove them. This
> - * might lead to split and split ranges are put in ced->mem.ranges[] array
> + * might lead to split and split ranges are put in cmem->ranges[] array
>   */
> -static int elf_header_exclude_ranges(struct crash_elf_data *ced)
> +static int elf_header_exclude_ranges(struct crash_mem *cmem)
>  {
> -	struct crash_mem *cmem = &ced->mem;
>  	int ret = 0;
>  
>  	/* Exclude crashkernel region */
> @@ -337,8 +319,7 @@ static int elf_header_exclude_ranges(struct crash_elf_data *ced)
>  
>  static int prepare_elf64_ram_headers_callback(struct resource *res, void *arg)
>  {
> -	struct crash_elf_data *ced = arg;
> -	struct crash_mem *cmem = &ced->mem;
> +	struct crash_mem *cmem = arg;
>  
>  	cmem->ranges[cmem->nr_ranges].start = res->start;
>  	cmem->ranges[cmem->nr_ranges].end = res->end;
> @@ -347,7 +328,7 @@ static int prepare_elf64_ram_headers_callback(struct resource *res, void *arg)
>  	return 0;
>  }
>  
> -static int prepare_elf64_headers(struct crash_elf_data *ced, int kernel_map,
> +static int prepare_elf64_headers(struct crash_mem *cmem, int kernel_map,
>  		void **addr, unsigned long *sz)
>  {
>  	Elf64_Ehdr *ehdr;
> @@ -356,12 +337,11 @@ static int prepare_elf64_headers(struct crash_elf_data *ced, int kernel_map,
>  	unsigned char *buf, *bufp;
>  	unsigned int cpu, i;
>  	unsigned long long notes_addr;
> -	struct crash_mem *cmem = &ced->mem;
>  	unsigned long mstart, mend;
>  
>  	/* extra phdr for vmcoreinfo elf note */
>  	nr_phdr = nr_cpus + 1;
> -	nr_phdr += ced->max_nr_ranges;
> +	nr_phdr += cmem->nr_ranges;
>  
>  	/*
>  	 * kexec-tools creates an extra PT_LOAD phdr for kernel text mapping
> @@ -455,29 +435,27 @@ static int prepare_elf64_headers(struct crash_elf_data *ced, int kernel_map,
>  static int prepare_elf_headers(struct kimage *image, void **addr,
>  					unsigned long *sz)
>  {
> -	struct crash_elf_data *ced;
> +	struct crash_mem *cmem;
>  	Elf64_Ehdr *ehdr;
>  	Elf64_Phdr *phdr;
>  	int ret, i;
>  
> -	ced = kzalloc(sizeof(*ced), GFP_KERNEL);
> -	if (!ced)
> +	cmem = fill_up_crash_elf_data();
> +	if (!cmem)
>  		return -ENOMEM;
>  
> -	fill_up_crash_elf_data(ced, image);
> -
> -	ret = walk_system_ram_res(0, -1, ced,
> +	ret = walk_system_ram_res(0, -1, cmem,
>  				prepare_elf64_ram_headers_callback);
>  	if (ret)
>  		goto out;
>  
>  	/* Exclude unwanted mem ranges */
> -	ret = elf_header_exclude_ranges(ced);
> +	ret = elf_header_exclude_ranges(cmem);
>  	if (ret)
>  		goto out;
>  
>  	/* By default prepare 64bit headers */
> -	ret =  prepare_elf64_headers(ced,
> +	ret =  prepare_elf64_headers(cmem,
>  				(int)IS_ENABLED(CONFIG_X86_64), addr, sz);
>  	if (ret)
>  		goto out;
> @@ -496,7 +474,7 @@ static int prepare_elf_headers(struct kimage *image, void **addr,
>  			break;
>  		}
>  out:
> -	kfree(ced);
> +	vfree(cmem);
>  	return ret;
>  }
>  
> -- 
> 2.16.2
> 

Thanks
Dave