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Date:   Wed, 12 Jul 2023 09:31:31 +0900
From:   Masami Hiramatsu (Google) <mhiramat@kernel.org>
To:     Alessandro Carminati <alessandro.carminati@gmail.com>
Cc:     rostedt@goodmis.org, mhiramat@kernel.org,
        linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org,
        bristot@kernel.org, vmalik@redhat.com,
        Masahiro Yamada <masahiroy@kernel.org>,
        linux-kbuild@vger.kernel.org
Subject: Re: [PATCH] scripts/link-vmlinux.sh: Add alias to duplicate symbols
 for kallsyms
Message-Id: <20230712093131.7832fdf6b7130328e58e85b6@kernel.org>
In-Reply-To: <20230711151925.1092080-1-alessandro.carminati@gmail.com>
References: <20230711151925.1092080-1-alessandro.carminati@gmail.com>
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On Tue, 11 Jul 2023 15:19:25 +0000
Alessandro Carminati <alessandro.carminati@gmail.com> wrote:

> It is not uncommon for drivers or modules related to similar peripherals
> to have symbols with the exact same name.
> While this is not a problem for the kernel's binary itself, it becomes an
> issue when attempting to trace or probe specific functions using
> infrastructure like ftrace or kprobe.
> 
> The tracing subsystem relies on the `nm -n vmlinux` output, which provides
> symbol information from the kernel's ELF binary. However, when multiple
> symbols share the same name, the standard nm output does not differentiate
> between them. This can lead to confusion and difficulty when trying to
> probe the intended symbol.
> 
> ~ # cat /proc/kallsyms | grep " name_show"
> ffffffff8c4f76d0 t name_show
> ffffffff8c9cccb0 t name_show
> ffffffff8cb0ac20 t name_show
> ffffffff8cc728c0 t name_show
> ffffffff8ce0efd0 t name_show
> ffffffff8ce126c0 t name_show
> ffffffff8ce1dd20 t name_show
> ffffffff8ce24e70 t name_show
> ffffffff8d1104c0 t name_show
> ffffffff8d1fe480 t name_show
> 
> **kas_alias** addresses this challenge by extending the symbol names with
> unique suffixes during the kernel build process.
> The newly created aliases for these duplicated symbols are unique names
> that can be fed to the tracefs interface. By doing so, it enables
> previously unreachable symbols to be probed.
> 
> ~ # cat /proc/kallsyms | grep " name_show"
> ffffffff974f76d0 t name_show
> ffffffff974f76d0 t name_show__alias__6340
> ffffffff979cccb0 t name_show
> ffffffff979cccb0 t name_show__alias__6341
> ffffffff97b0ac20 t name_show
> ffffffff97b0ac20 t name_show__alias__6342
> ffffffff97c728c0 t name_show
> ffffffff97c728c0 t name_show__alias__6343
> ffffffff97e0efd0 t name_show
> ffffffff97e0efd0 t name_show__alias__6344
> ffffffff97e126c0 t name_show
> ffffffff97e126c0 t name_show__alias__6345
> ffffffff97e1dd20 t name_show
> ffffffff97e1dd20 t name_show__alias__6346
> ffffffff97e24e70 t name_show
> ffffffff97e24e70 t name_show__alias__6347
> ffffffff981104c0 t name_show
> ffffffff981104c0 t name_show__alias__6348
> ffffffff981fe480 t name_show
> ffffffff981fe480 t name_show__alias__6349
> ~ # echo "p:kprobes/evnt1 name_show__alias__6349" \
> > >/sys/kernel/tracing/kprobe_events
> ~ # cat /sys/kernel/tracing/kprobe_events
> p:kprobes/evnt1 name_show__alias__6349

This is a nice feature. I found one issue with padding symbols (and CFI) With
those options, kernel will make '__cfi_' or '___pfx_' prefixed symbols for each
function on some arch. But those generated symbols are not executed but for
padding or storing integrity data. So you don't need to make aliases for those
symbols.

ffffffff8102c770 t __cfi_event_show
ffffffff8102c770 t __cfi_event_show__alias__498
ffffffff8102c780 t event_show
ffffffff8102c780 t event_show__alias__1505
ffffffff8102c7b0 t __cfi_umask_show
ffffffff8102c7b0 t __cfi_umask_show__alias__736
ffffffff8102c7c0 t umask_show
ffffffff8102c7c0 t umask_show__alias__4217
ffffffff8102c7f0 t __cfi_edge_show
ffffffff8102c7f0 t __cfi_edge_show__alias__484
ffffffff8102c800 t edge_show
ffffffff8102c800 t edge_show__alias__1478
ffffffff8102c830 t __cfi_inv_show
ffffffff8102c830 t __cfi_inv_show__alias__528
ffffffff8102c840 t inv_show
ffffffff8102c840 t inv_show__alias__1789
ffffffff8102c870 t __cfi_cmask_show
ffffffff8102c870 t __cfi_cmask_show__alias__439
ffffffff8102c880 t cmask_show
ffffffff8102c880 t cmask_show__alias__1257

Can you skip aliases for __cfi_ or __pfx_ symbols?

Thank you,

> 
> Signed-off-by: Alessandro Carminati (Red Hat) <alessandro.carminati@gmail.com>
> ---
>  init/Kconfig                        |  17 ++
>  scripts/Makefile                    |   4 +
>  scripts/kas_alias/Makefile          |   4 +
>  scripts/kas_alias/duplicates_list.c |  70 +++++++++
>  scripts/kas_alias/duplicates_list.h |  15 ++
>  scripts/kas_alias/item_list.c       | 230 ++++++++++++++++++++++++++++
>  scripts/kas_alias/item_list.h       |  26 ++++
>  scripts/kas_alias/kas_alias.c       | 112 ++++++++++++++
>  scripts/link-vmlinux.sh             |   6 +-
>  9 files changed, 483 insertions(+), 1 deletion(-)
>  create mode 100644 scripts/kas_alias/Makefile
>  create mode 100644 scripts/kas_alias/duplicates_list.c
>  create mode 100644 scripts/kas_alias/duplicates_list.h
>  create mode 100644 scripts/kas_alias/item_list.c
>  create mode 100644 scripts/kas_alias/item_list.h
>  create mode 100644 scripts/kas_alias/kas_alias.c
> 
> diff --git a/init/Kconfig b/init/Kconfig
> index f7f65af4ee12..a67b7b1c604b 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1737,6 +1737,23 @@ config KALLSYMS_BASE_RELATIVE
>  	  time constants, and no relocation pass is required at runtime to fix
>  	  up the entries based on the runtime load address of the kernel.
>  
> +config KALLSYMS_ALIAS
> +	bool "Produces alias for duplicated symbols" if EXPERT
> +	depends on KALLSYMS
> +	help
> +	  It is not uncommon for drivers or modules related to similar
> +	  peripherals to have symbols with the exact same name.
> +	  While this is not a problem for the kernel's binary itself, it
> +	  becomes an issue when attempting to trace or probe specific
> +	  functions using infrastructure like ftrace or kprobe.
> +
> +	  This option addresses this challenge by extending the symbol names
> +	  with unique suffixes during the kernel build process.
> +	  The newly created aliases for these duplicated symbols are unique
> +	  names that can be fed to the ftrace sysfs interface. By doing so, it
> +	  enables previously unreachable symbols to be probed.
> +
> +
>  # end of the "standard kernel features (expert users)" menu
>  
>  # syscall, maps, verifier
> diff --git a/scripts/Makefile b/scripts/Makefile
> index 32b6ba722728..65fafe17cfe5 100644
> --- a/scripts/Makefile
> +++ b/scripts/Makefile
> @@ -49,3 +49,7 @@ subdir-$(CONFIG_SECURITY_SELINUX) += selinux
>  
>  # Let clean descend into subdirs
>  subdir-	+= basic dtc gdb kconfig mod
> +
> +# KALLSyms alias
> +subdir-$(CONFIG_KALLSYMS_ALIAS) += kas_alias
> +
> diff --git a/scripts/kas_alias/Makefile b/scripts/kas_alias/Makefile
> new file mode 100644
> index 000000000000..523fa3441013
> --- /dev/null
> +++ b/scripts/kas_alias/Makefile
> @@ -0,0 +1,4 @@
> +# SPDX-License-Identifier: GPL-2.0
> +hostprogs-always-$(CONFIG_KALLSYMS_ALIAS)    += kas_alias
> +
> +kas_alias-objs        := duplicates_list.o item_list.o kas_alias.o
> diff --git a/scripts/kas_alias/duplicates_list.c b/scripts/kas_alias/duplicates_list.c
> new file mode 100644
> index 000000000000..e7a3d2917937
> --- /dev/null
> +++ b/scripts/kas_alias/duplicates_list.c
> @@ -0,0 +1,70 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +#include <stdint.h>
> +#include <stdio.h>
> +#include <string.h>
> +#include <stdlib.h>
> +#include <stdbool.h>
> +
> +#include "item_list.h"
> +#include "duplicates_list.h"
> +
> +struct duplicate_item *find_duplicates(struct item *list)
> +{
> +	struct duplicate_item *current_duplicate = NULL;
> +	struct duplicate_item *duplicates = NULL;
> +	struct duplicate_item *new_duplicate;
> +	struct item *current_item = list;
> +	bool prev_was_duplicate = false;
> +	struct item *prev_item = NULL;
> +
> +	while (current_item) {
> +		if ((prev_item && (strcmp(current_item->symb_name, prev_item->symb_name) == 0)) ||
> +		    prev_was_duplicate) {
> +			if (!duplicates) {
> +				duplicates = malloc(sizeof(struct duplicate_item));
> +				if (!duplicates)
> +					return NULL;
> +
> +				duplicates->original_item = prev_item;
> +				duplicates->next = NULL;
> +				current_duplicate = duplicates;
> +			} else {
> +				new_duplicate = malloc(sizeof(struct duplicate_item));
> +				if (!new_duplicate) {
> +					free_duplicates(&duplicates);
> +					return NULL;
> +				}
> +
> +				new_duplicate->original_item = prev_item;
> +				new_duplicate->next = NULL;
> +				current_duplicate->next = new_duplicate;
> +				current_duplicate = new_duplicate;
> +
> +				if ((strcmp(current_item->symb_name, prev_item->symb_name) != 0) &&
> +				    (prev_was_duplicate))
> +					prev_was_duplicate = false;
> +				else
> +					prev_was_duplicate = true;
> +			}
> +		}
> +
> +		prev_item = current_item;
> +		current_item = current_item->next;
> +	}
> +
> +	return duplicates;
> +}
> +
> +void free_duplicates(struct duplicate_item **duplicates)
> +{
> +	struct duplicate_item *duplicates_iterator = *duplicates;
> +	struct duplicate_item *app;
> +
> +	while (duplicates_iterator) {
> +		app = duplicates_iterator;
> +		duplicates_iterator = duplicates_iterator->next;
> +		free(app);
> +	}
> +
> +	*duplicates = NULL;
> +}
> diff --git a/scripts/kas_alias/duplicates_list.h b/scripts/kas_alias/duplicates_list.h
> new file mode 100644
> index 000000000000..76aa73e584bc
> --- /dev/null
> +++ b/scripts/kas_alias/duplicates_list.h
> @@ -0,0 +1,15 @@
> +/* SPDX-License-Identifier: GPL-2.0-or-later */
> +#ifndef DUPLICATES_LIST_H
> +#define DUPLICATES_LIST_H
> +
> +#include "item_list.h"
> +
> +struct duplicate_item {
> +	struct item *original_item;
> +	struct duplicate_item *next;
> +};
> +
> +struct duplicate_item *find_duplicates(struct item *list);
> +void free_duplicates(struct duplicate_item **duplicates);
> +
> +#endif
> diff --git a/scripts/kas_alias/item_list.c b/scripts/kas_alias/item_list.c
> new file mode 100644
> index 000000000000..7c9d5aecca9a
> --- /dev/null
> +++ b/scripts/kas_alias/item_list.c
> @@ -0,0 +1,230 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <stdint.h>
> +#include <string.h>
> +#include <stdbool.h>
> +#include <assert.h>
> +#include "item_list.h"
> +
> +#define CHECK_ORDER_BY_ADDRESS(sort_by, current, temp, op) \
> +	((sort_by) == BY_ADDRESS && (current)->addr op (temp)->addr)
> +#define CHECK_ORDER_BY_NAME(sort_by, current, temp, op) \
> +	((sort_by) == BY_NAME && strcmp((current)->symb_name, (temp)->symb_name) op 0)
> +
> +struct item *list_index[96] = {0};
> +
> +void build_index(struct item *list)
> +{
> +	char current_first_letter = ' ';
> +	struct item *current = list;
> +
> +	while (current) {
> +		if (current->symb_name[0] != current_first_letter) {
> +			current_first_letter = current->symb_name[0];
> +			list_index[current_first_letter - 32] = current;
> +		}
> +		current = current->next;
> +	}
> +}
> +
> +struct item *add_item(struct item **list, const char *name, char stype, uint64_t addr)
> +{
> +	struct item *new_item;
> +	struct item *current;
> +
> +	new_item = malloc(sizeof(struct item));
> +	if (!new_item)
> +		return NULL;
> +
> +	strncpy(new_item->symb_name, name, MAX_NAME_SIZE);
> +	new_item->symb_name[MAX_NAME_SIZE - 1] = '\0';
> +	new_item->addr = addr;
> +	new_item->stype = stype;
> +	new_item->next = NULL;
> +
> +	if (!(*list)) {
> +		*list = new_item;
> +	} else {
> +		current = *list;
> +		while (current->next)
> +			current = current->next;
> +
> +		current->next = new_item;
> +	}
> +	return new_item;
> +}
> +
> +void sort_list(struct item **list, int sort_by)
> +{
> +	struct item *current = *list;
> +	struct item *sorted = NULL;
> +	struct item *next_item;
> +	struct item *temp;
> +
> +	if (!(*list) || !((*list)->next))
> +		return;
> +
> +	while (current) {
> +		next_item = current->next;
> +		if (!sorted ||
> +		   (CHECK_ORDER_BY_ADDRESS(sort_by, current, sorted, <) ||
> +		    CHECK_ORDER_BY_NAME(sort_by, current, sorted, >=))) {
> +			current->next = sorted;
> +			sorted = current;
> +		} else {
> +			temp = sorted;
> +			while (temp->next &&
> +			      (CHECK_ORDER_BY_ADDRESS(sort_by, current, temp->next, >=) ||
> +			       CHECK_ORDER_BY_NAME(sort_by, current, temp->next, >=)))
> +				temp = temp->next;
> +
> +			current->next = temp->next;
> +			temp->next = current;
> +		}
> +		current = next_item;
> +	}
> +
> +	*list = sorted;
> +}
> +
> +struct item *merge(struct item *left, struct item *right, int sort_by)
> +{
> +	struct item *current = NULL;
> +	struct item *result = NULL;
> +
> +	if (!left)
> +		return right;
> +	if (!right)
> +		return left;
> +
> +	if (sort_by == BY_NAME) {
> +		if (strcmp(left->symb_name, right->symb_name) <= 0) {
> +			result = left;
> +			left = left->next;
> +		} else {
> +			result = right;
> +			right = right->next;
> +		}
> +	} else {
> +		if (sort_by == BY_ADDRESS) {
> +			if (left->addr <= right->addr) {
> +				result = left;
> +				left = left->next;
> +			} else {
> +				result = right;
> +				right = right->next;
> +			}
> +		}
> +	}
> +
> +	current = result;
> +
> +	while (left && right) {
> +		if (sort_by == BY_NAME) {
> +			if (strcmp(left->symb_name, right->symb_name) <= 0) {
> +				current->next = left;
> +				left = left->next;
> +			} else {
> +				current->next = right;
> +				right = right->next;
> +			}
> +		} else {
> +			if (sort_by == BY_ADDRESS) {
> +				if (left->addr <= right->addr) {
> +					current->next = left;
> +					left = left->next;
> +				} else {
> +					current->next = right;
> +					right = right->next;
> +				}
> +			}
> +		}
> +
> +		current = current->next;
> +	}
> +
> +	if (left) {
> +		current->next = left;
> +	} else {
> +		if (right)
> +			current->next = right;
> +	}
> +
> +	return result;
> +}
> +
> +struct item *merge_sort(struct item *head, int sort_by)
> +{
> +	struct item *right;
> +	struct item *slow;
> +	struct item *fast;
> +	struct item *left;
> +
> +	if (!head || !head->next)
> +		return head;
> +
> +	slow = head;
> +	fast = head->next;
> +
> +	while (fast && fast->next) {
> +		slow = slow->next;
> +		fast = fast->next->next;
> +	}
> +
> +	left = head;
> +	right = slow->next;
> +	slow->next = NULL;
> +
> +	left = merge_sort(left, sort_by);
> +	right = merge_sort(right, sort_by);
> +
> +	return merge(left, right, sort_by);
> +}
> +
> +void sort_list_m(struct item **head, int sort_by)
> +{
> +	if (!(*head) || !((*head)->next))
> +		return;
> +
> +	*head = merge_sort(*head, sort_by);
> +}
> +
> +int insert_after(struct item *list, const uint64_t search_addr,
> +		 const char *name, uint64_t addr, char stype)
> +{
> +	struct item *new_item;
> +	struct item *current;
> +	int ret = 0;
> +
> +	current = (list_index[name[0] - 32]) ? list_index[name[0] - 32] : list;
> +	while (current) {
> +		if (current->addr == search_addr) {
> +			new_item = malloc(sizeof(struct item));
> +			if (!new_item)
> +				return ret;
> +			strncpy(new_item->symb_name, name, MAX_NAME_SIZE);
> +			new_item->symb_name[MAX_NAME_SIZE - 1] = '\0';
> +			new_item->addr = addr;
> +			new_item->stype = stype;
> +			new_item->next = current->next;
> +			current->next = new_item;
> +			ret = 1;
> +			break;
> +		}
> +		current = current->next;
> +	}
> +	return ret;
> +}
> +
> +void free_items(struct item **head)
> +{
> +	struct item *app, *item_iterator = *head;
> +
> +	while (item_iterator) {
> +		app = item_iterator;
> +		item_iterator = item_iterator->next;
> +		free(app);
> +	}
> +	*head = NULL;
> +}
> diff --git a/scripts/kas_alias/item_list.h b/scripts/kas_alias/item_list.h
> new file mode 100644
> index 000000000000..b4891cb088ee
> --- /dev/null
> +++ b/scripts/kas_alias/item_list.h
> @@ -0,0 +1,26 @@
> +/* SPDX-License-Identifier: GPL-2.0-or-later */
> +#ifndef ITEM_LIST_H
> +#define ITEM_LIST_H
> +#include <stdint.h>
> +
> +#define MAX_NAME_SIZE 256
> +#define BY_ADDRESS 1
> +#define BY_NAME 2
> +
> +struct item {
> +	char		symb_name[MAX_NAME_SIZE];
> +	uint64_t	addr;
> +	char		stype;
> +	struct item	*next;
> +};
> +
> +void build_index(struct item *list);
> +struct item *add_item(struct item **list, const char *name, char stype, uint64_t addr);
> +void sort_list(struct item **list, int sort_by);
> +struct item *merge(struct item *left, struct item *right, int sort_by);
> +struct item *merge_sort(struct item *head, int sort_by);
> +void sort_list_m(struct item **head, int sort_by);
> +int insert_after(struct item *list, const uint64_t search_addr,
> +		 const char *name, uint64_t addr, char stype);
> +void free_items(struct item **head);
> +#endif
> diff --git a/scripts/kas_alias/kas_alias.c b/scripts/kas_alias/kas_alias.c
> new file mode 100644
> index 000000000000..d47cfec9f1f3
> --- /dev/null
> +++ b/scripts/kas_alias/kas_alias.c
> @@ -0,0 +1,112 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <stdint.h>
> +#include <string.h>
> +#include <stdbool.h>
> +#include <stdarg.h>
> +
> +#include "item_list.h"
> +#include "duplicates_list.h"
> +
> +int suffix_serial;
> +
> +static inline void verbose_msg(bool verbose, const char *fmt, ...)
> +{
> +	va_list args;
> +
> +	va_start(args, fmt);
> +	if (verbose)
> +		printf(fmt, args);
> +
> +	va_end(args);
> +}
> +
> +static void create_suffix(const char *name, char *output_suffix)
> +{
> +	sprintf(output_suffix, "%s__alias__%d", name, suffix_serial++);
> +}
> +
> +int main(int argc, char *argv[])
> +{
> +	char t, sym_name[MAX_NAME_SIZE], new_name[MAX_NAME_SIZE + 15];
> +	struct duplicate_item  *duplicate_iterator;
> +	struct duplicate_item *duplicate;
> +	struct item *head = {NULL};
> +	bool need_2_process = true;
> +	struct item *last = {NULL};
> +	struct item  *current;
> +	int verbose_mode = 0;
> +	uint64_t address;
> +	FILE *fp;
> +
> +	if (argc < 2 || argc > 3) {
> +		printf("Usage: %s <nmfile> [-verbose]\n", argv[0]);
> +		return 1;
> +	}
> +
> +	if (argc == 3 && strcmp(argv[2], "-verbose") == 0)
> +		verbose_mode = 1;
> +
> +	verbose_msg(verbose_mode, "Scanning nm data(%s)\n", argv[1]);
> +
> +	fp = fopen(argv[1], "r");
> +	if (!fp) {
> +		printf("Can't open input file.\n");
> +		return 1;
> +	}
> +
> +	while (fscanf(fp, "%lx %c %99s\n", &address, &t, sym_name) == 3) {
> +		if (strstr(sym_name, "__alias__1") != NULL) {
> +			if (verbose_mode && need_2_process)
> +				printf("Already processed\n");
> +			need_2_process = false;
> +			}
> +		last = add_item(&last, sym_name, t, address);
> +		if (!last) {
> +			printf("Error in allocate memory\n");
> +			free_items(&head);
> +			return 1;
> +		}
> +
> +		if (!head)
> +			head = last;
> +	}
> +
> +	fclose(fp);
> +
> +	if (need_2_process) {
> +		verbose_msg(verbose_mode, "Sorting nm data\n");
> +		sort_list_m(&head, BY_NAME);
> +		verbose_msg(verbose_mode, "Scanning nm data for duplicates\n");
> +		duplicate = find_duplicates(head);
> +		if (!duplicate) {
> +			printf("Error in duplicates list\n");
> +			return 1;
> +		}
> +
> +		verbose_msg(verbose_mode, "Applying suffixes\n");
> +		build_index(head);
> +		duplicate_iterator = duplicate;
> +		while (duplicate_iterator) {
> +			create_suffix(duplicate_iterator->original_item->symb_name, new_name);
> +			if (!insert_after(head, duplicate_iterator->original_item->addr, new_name,
> +					  duplicate_iterator->original_item->addr,
> +					  duplicate_iterator->original_item->stype))
> +				return 1;
> +			duplicate_iterator = duplicate_iterator->next;
> +		}
> +
> +		sort_list_m(&head, BY_ADDRESS);
> +	}
> +	current = head;
> +	while (current) {
> +		printf("%08lx %c %s\n", current->addr, current->stype, current->symb_name);
> +		current = current->next;
> +	}
> +
> +	free_items(&head);
> +	free_duplicates(&duplicate);
> +
> +	return 0;
> +}
> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> index a432b171be82..4e3dea3ac2df 100755
> --- a/scripts/link-vmlinux.sh
> +++ b/scripts/link-vmlinux.sh
> @@ -161,7 +161,11 @@ kallsyms()
>  	fi
>  
>  	info KSYMS ${2}
> -	scripts/kallsyms ${kallsymopt} ${1} > ${2}
> +	if is_enabled CONFIG_KALLSYMS_ALIAS; then
> +		ALIAS=".alias"
> +		scripts/kas_alias/kas_alias ${1} >${1}${ALIAS}
> +		fi
> +	scripts/kallsyms ${kallsymopt} ${1}${ALIAS} > ${2}
>  }
>  
>  # Perform one step in kallsyms generation, including temporary linking of
> -- 
> 2.34.1
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>