scripts/sorttable: Zero out weak functions in mcount_loc table

When a function is annotated as "weak" and is overridden, the code is not
removed. If it is traced, the fentry/mcount location in the weak function
will be referenced by the "__mcount_loc" section. This will then be added
to the available_filter_functions list. Since only the address of the
functions are listed, to find the name to show, a search of kallsyms is
used.

Since kallsyms will return the function by simply finding the function
that the address is after but before the next function, an address of a
weak function will show up as the function before it. This is because
kallsyms does not save names of weak functions. This has caused issues in
the past, as now the traced weak function will be listed in
available_filter_functions with the name of the function before it.

At best, this will cause the previous function's name to be listed twice.
At worse, if the previous function was marked notrace, it will now show up
as a function that can be traced. Note that it only shows up that it can
be traced but will not be if enabled, which causes confusion.

 https://lore.kernel.org/all/20220412094923.0abe90955e5db486b7bca279@kernel.org/

The commit b39181f7c6907 ("ftrace: Add FTRACE_MCOUNT_MAX_OFFSET to avoid
adding weak function") was a workaround to this by checking the function
address before printing its name. If the address was too far from the
function given by the name then instead of printing the name it would
print: __ftrace_invalid_address___<invalid-offset>

The real issue is that these invalid addresses are listed in the ftrace
table look up which available_filter_functions is derived from. A place
holder must be listed in that file because set_ftrace_filter may take a
series of indexes into that file instead of names to be able to do O(1)
lookups to enable filtering (many tools use this method).

Even if kallsyms saved the size of the function, it does not remove the
need of having these place holders. The real solution is to not add a weak
function into the ftrace table in the first place.

To solve this, the sorttable.c code that sorts the mcount regions during
the build is modified to take a "nm -S vmlinux" input, sort it, and any
function listed in the mcount_loc section that is not within a boundary of
the function list given by nm is considered a weak function and is zeroed
out.

Note, this does not mean they will remain zero when booting as KASLR
will still shift those addresses. To handle this, the entries in the
mcount_loc section will be ignored if they are zero or match the
kaslr_offset() value.

Before:

 ~# grep __ftrace_invalid_address___ /sys/kernel/tracing/available_filter_functions | wc -l
 551

After:

 ~# grep __ftrace_invalid_address___ /sys/kernel/tracing/available_filter_functions | wc -l
 0

Cc: bpf <bpf@vger.kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nicolas Schier <nicolas@fjasle.eu>
Cc: Zheng Yejian <zhengyejian1@huawei.com>
Cc: Martin  Kelly <martin.kelly@crowdstrike.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Link: https://lore.kernel.org/20250218200022.883095980@goodmis.org
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 728ecda6e8d4dd48c7e784dd95b1f8b6fbd6d166..e3f89924f603e3eafaef434d3e1660d804226c79 100644 (file)
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -7004,6 +7004,7 @@ static int ftrace_process_locs(struct module *mod,
        unsigned long count;
        unsigned long *p;
        unsigned long addr;
+       unsigned long kaslr;
        unsigned long flags = 0; /* Shut up gcc */
        int ret = -ENOMEM;
 
@@ -7052,6 +7053,9 @@ static int ftrace_process_locs(struct module *mod,
                ftrace_pages->next = start_pg;
        }
 
+       /* For zeroed locations that were shifted for core kernel */
+       kaslr = !mod ? kaslr_offset() : 0;
+
        p = start;
        pg = start_pg;
        while (p < end) {
@@ -7063,7 +7067,7 @@ static int ftrace_process_locs(struct module *mod,
                 * object files to satisfy alignments.
                 * Skip any NULL pointers.
                 */
-               if (!addr) {
+               if (!addr || addr == kaslr) {
                        skipped++;
                        continue;
                }

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index 56a077d204cfa1b67609cf0ea4a8ebe5f5884f03..59b07fe6fd0040d26362b611c8f506b9340bac50 100755 (executable)
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -177,12 +177,14 @@ mksysmap()
 
 sorttable()
 {
-       ${objtree}/scripts/sorttable ${1}
+       ${NM} -S ${1} > .tmp_vmlinux.nm-sort
+       ${objtree}/scripts/sorttable -s .tmp_vmlinux.nm-sort ${1}
 }
 
 cleanup()
 {
        rm -f .btf.*
+       rm -f .tmp_vmlinux.nm-sort
        rm -f System.map
        rm -f vmlinux
        rm -f vmlinux.map

diff --git a/scripts/sorttable.c b/scripts/sorttable.c
index ec02a2852efb2cd57eacf01251c40485c02a3a52..23c7e0e6c0242172aec5903b5337dadaffd637bf 100644 (file)
--- a/scripts/sorttable.c
+++ b/scripts/sorttable.c
@@ -580,6 +580,98 @@ static void rela_write_addend(Elf_Rela *rela, uint64_t val)
        e.rela_write_addend(rela, val);
 }
 
+struct func_info {
+       uint64_t        addr;
+       uint64_t        size;
+};
+
+/* List of functions created by: nm -S vmlinux */
+static struct func_info *function_list;
+static int function_list_size;
+
+/* Allocate functions in 1k blocks */
+#define FUNC_BLK_SIZE  1024
+#define FUNC_BLK_MASK  (FUNC_BLK_SIZE - 1)
+
+static int add_field(uint64_t addr, uint64_t size)
+{
+       struct func_info *fi;
+       int fsize = function_list_size;
+
+       if (!(fsize & FUNC_BLK_MASK)) {
+               fsize += FUNC_BLK_SIZE;
+               fi = realloc(function_list, fsize * sizeof(struct func_info));
+               if (!fi)
+                       return -1;
+               function_list = fi;
+       }
+       fi = &function_list[function_list_size++];
+       fi->addr = addr;
+       fi->size = size;
+       return 0;
+}
+
+/* Only return match if the address lies inside the function size */
+static int cmp_func_addr(const void *K, const void *A)
+{
+       uint64_t key = *(const uint64_t *)K;
+       const struct func_info *a = A;
+
+       if (key < a->addr)
+               return -1;
+       return key >= a->addr + a->size;
+}
+
+/* Find the function in function list that is bounded by the function size */
+static int find_func(uint64_t key)
+{
+       return bsearch(&key, function_list, function_list_size,
+                      sizeof(struct func_info), cmp_func_addr) != NULL;
+}
+
+static int cmp_funcs(const void *A, const void *B)
+{
+       const struct func_info *a = A;
+       const struct func_info *b = B;
+
+       if (a->addr < b->addr)
+               return -1;
+       return a->addr > b->addr;
+}
+
+static int parse_symbols(const char *fname)
+{
+       FILE *fp;
+       char addr_str[20]; /* Only need 17, but round up to next int size */
+       char size_str[20];
+       char type;
+
+       fp = fopen(fname, "r");
+       if (!fp) {
+               perror(fname);
+               return -1;
+       }
+
+       while (fscanf(fp, "%16s %16s %c %*s\n", addr_str, size_str, &type) == 3) {
+               uint64_t addr;
+               uint64_t size;
+
+               /* Only care about functions */
+               if (type != 't' && type != 'T' && type != 'W')
+                       continue;
+
+               addr = strtoull(addr_str, NULL, 16);
+               size = strtoull(size_str, NULL, 16);
+               if (add_field(addr, size) < 0)
+                       return -1;
+       }
+       fclose(fp);
+
+       qsort(function_list, function_list_size, sizeof(struct func_info), cmp_funcs);
+
+       return 0;
+}
+
 static pthread_t mcount_sort_thread;
 static bool sort_reloc;
 
@@ -752,6 +844,21 @@ static void *sort_mcount_loc(void *arg)
                goto out;
        }
 
+       /* zero out any locations not found by function list */
+       if (function_list_size) {
+               for (void *ptr = vals; ptr < vals + size; ptr += long_size) {
+                       uint64_t key;
+
+                       key = long_size == 4 ? r((uint32_t *)ptr) : r8((uint64_t *)ptr);
+                       if (!find_func(key)) {
+                               if (long_size == 4)
+                                       *(uint32_t *)ptr = 0;
+                               else
+                                       *(uint64_t *)ptr = 0;
+                       }
+               }
+       }
+
        compare_values = long_size == 4 ? compare_values_32 : compare_values_64;
 
        qsort(vals, count, long_size, compare_values);
@@ -801,6 +908,8 @@ static void get_mcount_loc(struct elf_mcount_loc *emloc, Elf_Shdr *symtab_sec,
                return;
        }
 }
+#else /* MCOUNT_SORT_ENABLED */
+static inline int parse_symbols(const char *fname) { return 0; }
 #endif
 
 static int do_sort(Elf_Ehdr *ehdr,
@@ -1256,14 +1365,29 @@ int main(int argc, char *argv[])
        int i, n_error = 0;  /* gcc-4.3.0 false positive complaint */
        size_t size = 0;
        void *addr = NULL;
+       int c;
+
+       while ((c = getopt(argc, argv, "s:")) >= 0) {
+               switch (c) {
+               case 's':
+                       if (parse_symbols(optarg) < 0) {
+                               fprintf(stderr, "Could not parse %s\n", optarg);
+                               return -1;
+                       }
+                       break;
+               default:
+                       fprintf(stderr, "usage: sorttable [-s nm-file] vmlinux...\n");
+                       return 0;
+               }
+       }
 
-       if (argc < 2) {
+       if ((argc - optind) < 1) {
                fprintf(stderr, "usage: sorttable vmlinux...\n");
                return 0;
        }
 
        /* Process each file in turn, allowing deep failure. */
-       for (i = 1; i < argc; i++) {
+       for (i = optind; i < argc; i++) {
                addr = mmap_file(argv[i], &size);
                if (!addr) {
                        ++n_error;