Date:   Mon, 18 Sep 2023 17:06:31 -0400
From:   Jack Brennen <jbrennen@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nicolas Schier <nicolas@...sle.eu>, Tom Rix <trix@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        llvm@...ts.linux.dev, Jack Brennen <jbrennen@...gle.com>
Subject: [PATCH] modpost: Optimize symbol search from linear to binary search

Modify modpost to use binary search for converting addresses back
into symbol references.  Previously it used linear search.

This change saves a few seconds of wall time for defconfig builds,
but can save several minutes on allyesconfigs.

Before:
$ make LLVM=1 -j128 allyesconfig vmlinux -s KCFLAGS="-Wno-error"
        Elapsed (wall clock) time (h:mm:ss or m:ss): 13:30.31

After:
$ make LLVM=1 -j128 allyesconfig vmlinux -s KCFLAGS="-Wno-error"
        Elapsed (wall clock) time (h:mm:ss or m:ss): 11:43.43

Signed-off-by: Jack Brennen <jbrennen@...gle.com>
Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
---
 scripts/mod/Makefile    |   4 +-
 scripts/mod/modpost.c   |  60 +----------
 scripts/mod/modpost.h   |  25 +++++
 scripts/mod/symsearch.c | 233 ++++++++++++++++++++++++++++++++++++++++
 4 files changed, 265 insertions(+), 57 deletions(-)
 create mode 100644 scripts/mod/symsearch.c

diff --git a/scripts/mod/Makefile b/scripts/mod/Makefile
index c9e38ad937fd..3c54125eb373 100644
--- a/scripts/mod/Makefile
+++ b/scripts/mod/Makefile
@@ -5,7 +5,7 @@ CFLAGS_REMOVE_empty.o += $(CC_FLAGS_LTO)
 hostprogs-always-y	+= modpost mk_elfconfig
 always-y		+= empty.o
 
-modpost-objs	:= modpost.o file2alias.o sumversion.o
+modpost-objs	:= modpost.o file2alias.o sumversion.o symsearch.o
 
 devicetable-offsets-file := devicetable-offsets.h
 
@@ -16,7 +16,7 @@ targets += $(devicetable-offsets-file) devicetable-offsets.s
 
 # dependencies on generated files need to be listed explicitly
 
-$(obj)/modpost.o $(obj)/file2alias.o $(obj)/sumversion.o: $(obj)/elfconfig.h
+$(obj)/modpost.o $(obj)/file2alias.o $(obj)/sumversion.o $(obj)/symsearch.o: $(obj)/elfconfig.h
 $(obj)/file2alias.o: $(obj)/$(devicetable-offsets-file)
 
 quiet_cmd_elfconfig = MKELF   $@
diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index de499dce5265..975f235aca2c 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -22,7 +22,6 @@
 #include <errno.h>
 #include "modpost.h"
 #include "../../include/linux/license.h"
-#include "../../include/linux/module_symbol.h"
 
 static bool module_enabled;
 /* Are we using CONFIG_MODVERSIONS? */
@@ -577,11 +576,14 @@ static int parse_elf(struct elf_info *info, const char *filename)
 			*p = TO_NATIVE(*p);
 	}
 
+	symsearch_init(info);
+
 	return 1;
 }
 
 static void parse_elf_finish(struct elf_info *info)
 {
+	symsearch_finish(info);
 	release_file(info->hdr, info->size);
 }
 
@@ -1039,65 +1041,13 @@ static int secref_whitelist(const char *fromsec, const char *fromsym,
 	return 1;
 }
 
-/*
- * If there's no name there, ignore it; likewise, ignore it if it's
- * one of the magic symbols emitted used by current tools.
- *
- * Otherwise if find_symbols_between() returns those symbols, they'll
- * fail the whitelist tests and cause lots of false alarms ... fixable
- * only by merging __exit and __init sections into __text, bloating
- * the kernel (which is especially evil on embedded platforms).
- */
-static inline int is_valid_name(struct elf_info *elf, Elf_Sym *sym)
-{
-	const char *name = elf->strtab + sym->st_name;
-
-	if (!name || !strlen(name))
-		return 0;
-	return !is_mapping_symbol(name);
-}
-
 /* Look up the nearest symbol based on the section and the address */
 static Elf_Sym *find_nearest_sym(struct elf_info *elf, Elf_Addr addr,
 				 unsigned int secndx, bool allow_negative,
 				 Elf_Addr min_distance)
 {
-	Elf_Sym *sym;
-	Elf_Sym *near = NULL;
-	Elf_Addr sym_addr, distance;
-	bool is_arm = (elf->hdr->e_machine == EM_ARM);
-
-	for (sym = elf->symtab_start; sym < elf->symtab_stop; sym++) {
-		if (get_secindex(elf, sym) != secndx)
-			continue;
-		if (!is_valid_name(elf, sym))
-			continue;
-
-		sym_addr = sym->st_value;
-
-		/*
-		 * For ARM Thumb instruction, the bit 0 of st_value is set
-		 * if the symbol is STT_FUNC type. Mask it to get the address.
-		 */
-		if (is_arm && ELF_ST_TYPE(sym->st_info) == STT_FUNC)
-			 sym_addr &= ~1;
-
-		if (addr >= sym_addr)
-			distance = addr - sym_addr;
-		else if (allow_negative)
-			distance = sym_addr - addr;
-		else
-			continue;
-
-		if (distance <= min_distance) {
-			min_distance = distance;
-			near = sym;
-		}
-
-		if (min_distance == 0)
-			break;
-	}
-	return near;
+	return symsearch_find_nearest(elf, addr, secndx,
+				      allow_negative, min_distance);
 }
 
 static Elf_Sym *find_fromsym(struct elf_info *elf, Elf_Addr addr,
diff --git a/scripts/mod/modpost.h b/scripts/mod/modpost.h
index 5f94c2c9f2d9..6413f26fcb6b 100644
--- a/scripts/mod/modpost.h
+++ b/scripts/mod/modpost.h
@@ -10,6 +10,7 @@
 #include <fcntl.h>
 #include <unistd.h>
 #include <elf.h>
+#include "../../include/linux/module_symbol.h"
 
 #include "list.h"
 #include "elfconfig.h"
@@ -128,6 +129,8 @@ struct elf_info {
 	 * take shndx from symtab_shndx_start[N] instead */
 	Elf32_Word   *symtab_shndx_start;
 	Elf32_Word   *symtab_shndx_stop;
+
+	struct symsearch *symsearch;
 };
 
 /* Accessor for sym->st_shndx, hides ugliness of "64k sections" */
@@ -154,6 +157,28 @@ static inline unsigned int get_secindex(const struct elf_info *info,
 	return index;
 }
 
+/*
+ * If there's no name there, ignore it; likewise, ignore it if it's
+ * one of the magic symbols emitted used by current tools.
+ *
+ * Internal symbols created by tools should be ignored by modpost.
+ */
+static inline int is_valid_name(struct elf_info *elf, Elf_Sym *sym)
+{
+	const char *name = elf->strtab + sym->st_name;
+
+	if (!name || !strlen(name))
+		return 0;
+	return !is_mapping_symbol(name);
+}
+
+/* symsearch.c */
+void symsearch_init(struct elf_info *elf);
+void symsearch_finish(struct elf_info *elf);
+Elf_Sym *symsearch_find_nearest(struct elf_info *elf, Elf_Addr addr,
+				unsigned int secndx, bool allow_negative,
+				Elf_Addr min_distance);
+
 /* file2alias.c */
 void handle_moddevtable(struct module *mod, struct elf_info *info,
 			Elf_Sym *sym, const char *symname);
diff --git a/scripts/mod/symsearch.c b/scripts/mod/symsearch.c
new file mode 100644
index 000000000000..aab79262512b
--- /dev/null
+++ b/scripts/mod/symsearch.c
@@ -0,0 +1,233 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/* Helper functions for finding the symbol in an ELF which is "nearest"
+ * to a given address.
+ */
+
+#include "modpost.h"
+
+/* Struct used for binary search. */
+struct syminfo {
+	unsigned int symbol_index;
+	unsigned int section_index;
+	Elf_Addr addr;
+};
+
+/* Container used to hold an entire binary search table.
+ * Entries in table are ascending, sorted first by section_index,
+ * then by addr, and last by symbol_index.  The sorting by
+ * symbol_index is used to duplicate the quirks of the prior
+ * find_nearest_sym() function, where exact matches to an address
+ * return the first symtab entry seen, but near misses return the
+ * last symtab entry seen.
+ * The first and last entries of the table are sentinels and their
+ * values only matter in two places:  when we sort the table, and
+ * on lookups, the end sentinel should not have an addr field which
+ * matches its immediate predecessor.  To meet these requirements,
+ * we initialize them to (0,0,0) and (max,max,max), and then after
+ * sorting, we tweak the end sentinel's addr field accordingly.
+ */
+struct symsearch {
+	size_t table_size;
+	struct syminfo table[];
+};
+
+static inline bool is_sym_searchable(struct elf_info *elf, Elf_Sym *sym)
+{
+	return is_valid_name(elf, sym) != 0;
+}
+
+static int syminfo_compare(const void *s1, const void *s2)
+{
+	const struct syminfo *sym1 = s1;
+	const struct syminfo *sym2 = s2;
+
+	if (sym1->section_index > sym2->section_index)
+		return 1;
+	if (sym1->section_index < sym2->section_index)
+		return -1;
+	if (sym1->addr > sym2->addr)
+		return 1;
+	if (sym1->addr < sym2->addr)
+		return -1;
+	if (sym1->symbol_index > sym2->symbol_index)
+		return 1;
+	if (sym1->symbol_index < sym2->symbol_index)
+		return -1;
+	return 0;
+}
+
+static size_t symbol_count(struct elf_info *elf)
+{
+	size_t result = 0;
+
+	for (Elf_Sym *sym = elf->symtab_start; sym < elf->symtab_stop; sym++) {
+		if (is_sym_searchable(elf, sym))
+			result++;
+	}
+	return result;
+}
+
+/* Populate the search array that we just allocated.
+ * Be slightly paranoid here.  If the ELF file changes during processing,
+ * or if the behavior of is_sym_searchable() changes during processing,
+ * we want to catch it; neither of those is acceptable.
+ */
+static void symsearch_populate(struct elf_info *elf,
+			       struct syminfo *table,
+			       size_t table_size)
+{
+	bool is_arm = (elf->hdr->e_machine == EM_ARM);
+
+	/* Start sentinel */
+	if (table_size-- == 0)
+		fatal("%s: size mismatch\n", __func__);
+	table->symbol_index = 0;
+	table->section_index = 0;
+	table->addr = 0;
+	table++;
+
+	for (Elf_Sym *sym = elf->symtab_start; sym < elf->symtab_stop; sym++) {
+		if (is_sym_searchable(elf, sym)) {
+			if (table_size-- == 0)
+				fatal("%s: size mismatch\n", __func__);
+			table->symbol_index = sym - elf->symtab_start;
+			table->section_index = get_secindex(elf, sym);
+			table->addr = sym->st_value;
+
+			/*
+			 * For ARM Thumb instruction, the bit 0 of st_value is
+			 * set if the symbol is STT_FUNC type. Mask it to get
+			 * the address.
+			 */
+			if (is_arm && ELF_ST_TYPE(sym->st_info) == STT_FUNC)
+				table->addr &= ~1;
+
+			table++;
+		}
+	}
+
+	/* End sentinel; all values are unsigned so -1 wraps to max */
+	if (table_size != 1)
+		fatal("%s: size mismatch\n", __func__);
+	table->symbol_index = -1;
+	table->section_index = -1;
+	table->addr = -1;
+}
+
+void symsearch_init(struct elf_info *elf)
+{
+	/* +2 here to allocate space for the start and end sentinels */
+	size_t table_size = symbol_count(elf) + 2;
+
+	elf->symsearch = NOFAIL(malloc(
+					sizeof(struct symsearch) +
+					sizeof(struct syminfo) * table_size));
+	elf->symsearch->table_size = table_size;
+
+	symsearch_populate(elf, elf->symsearch->table, table_size);
+	qsort(elf->symsearch->table, table_size,
+	      sizeof(struct syminfo), syminfo_compare);
+
+	/* A bit of paranoia; make sure that the end sentinel's address is
+	 * different than its predecessor.  Not doing this could cause
+	 * possible undefined behavior if anybody ever inserts a symbol
+	 * with section_index and addr both at their max values.
+	 * Doing this little bit of defensive programming is more efficient
+	 * than checking for array overruns later.
+	 */
+	elf->symsearch->table[table_size - 1].addr =
+		elf->symsearch->table[table_size - 2].addr + 1;
+}
+
+void symsearch_finish(struct elf_info *elf)
+{
+	free(elf->symsearch);
+	elf->symsearch = NULL;
+}
+
+/* Find the syminfo which is in secndx and "nearest" to addr.
+ * allow_negative: allow returning a symbol whose address is > addr.
+ * min_distance: ignore symbols which are further away than this.
+ *
+ * Returns a nonzero index into the symsearch table for success.
+ * Returns NULL if no legal symbol is found within the requested range.
+ */
+static size_t symsearch_find_impl(struct elf_info *elf, Elf_Addr addr,
+				  unsigned int secndx, bool allow_negative,
+				  Elf_Addr min_distance)
+{
+	/* Find the target in the array; it will lie between two elements.
+	 * Invariant here: table[lo] < target <= table[hi]
+	 * For the purposes of search, exact hits in the search array are
+	 * considered greater than the target.	This means that if we do
+	 * get an exact hit, then once the search terminates, table[hi]
+	 * will be the exact match which has the lowest symbol index.
+	 */
+	struct syminfo *table = elf->symsearch->table;
+	size_t hi = elf->symsearch->table_size - 1;
+	size_t lo = 0;
+	bool hi_is_usable = false;
+	bool lo_is_usable = false;
+	Elf_Addr hi_distance = -1;  // max Elf_Addr
+	Elf_Addr lo_distance = -1;  // max Elf_Addr
+	Elf_Addr min_distance_lo = min_distance;
+	Elf_Addr min_distance_hi = allow_negative ? min_distance : 0;
+
+	for (;;) {
+		size_t mid;
+
+		mid = lo + (hi - lo) / 2;
+		if (mid == lo)
+			break;
+		if (secndx > table[mid].section_index) {
+			lo = mid;
+		} else if (secndx < table[mid].section_index) {
+			hi = mid;
+		} else if (addr > table[mid].addr) {
+			lo = mid;
+			lo_distance = addr - table[mid].addr;
+			lo_is_usable = (lo_distance <= min_distance_lo);
+		} else {
+			hi = mid;
+			hi_distance = table[mid].addr - addr;
+			hi_is_usable = (hi_distance <= min_distance_hi);
+		}
+	}
+
+	if (hi_is_usable && lo_is_usable) {
+		lo_is_usable = (lo_distance <= hi_distance);
+		hi_is_usable = (hi_distance <= lo_distance);
+	}
+
+	if (!hi_is_usable)
+		return lo_is_usable ? lo : 0;
+
+	if (hi_distance == 0)
+		return hi;
+
+	/* Match quirks of existing behavior.  Advance hi to the last
+	 * matching entry in the search table.	We don't need to worry
+	 * about running off the end of the array due to the sentinel.
+	 */
+	while (table[hi+1].addr == table[hi].addr &&
+	       table[hi+1].section_index == table[hi].section_index) {
+		hi++;
+	}
+
+	return (lo_is_usable &&
+		table[lo].symbol_index > table[hi].symbol_index) ? lo : hi;
+}
+
+Elf_Sym *symsearch_find_nearest(struct elf_info *elf, Elf_Addr addr,
+				unsigned int secndx, bool allow_negative,
+				Elf_Addr min_distance)
+{
+	size_t result = symsearch_find_impl(elf, addr, secndx,
+					    allow_negative, min_distance);
+
+	if (result == 0)
+		return NULL;
+
+	return &elf->symtab_start[elf->symsearch->table[result].symbol_index];
+}
-- 
2.42.0.459.ge4e396fd5e-goog

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