On Tue, Sep 19, 2023 at 6:06 AM Jack Brennen <jbrennen@xxxxxxxxxx> wrote:
>
> 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.
Thanks.
Binary search is a good idea.
> Before:
> $ make LLVM=1 -j128 allyesconfig vmlinux -s KCFLAGS="-Wno-error"
> Elapsed (wall clock) time (h:mm:ss or m:ss): 13:30.31
Instead of the time for the entire build,
can you put the time for the modpost command?
If you allyesconfig case,
$ time scripts/mod/modpost -M -m -a -N -o vmlinux.symvers vmlinux.o
> 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.
> + */
>
Can you use the following block comment style?
/*
* Helper functions for finding the symbol in an ELF which is "nearest"
* to a given address.
*/
> +#include "modpost.h"
> +
> +/* Struct used for binary search. */
I think this obvious comment is unneeded.
> +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.
Preserving this quirk makes the code complicated.
I do not mind changing the behavior of the corner case.
> + * 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[];
> +};
syminfo::symbol_index is unsigned int.
symsearch::table_size is size_t.
symbol_index of the last element is always larger than
elf->symsearch->table_size.
So, the code works only within 32-bit width anyway.
> +
> +static inline bool is_sym_searchable(struct elf_info *elf, Elf_Sym *sym)
> +{
> + return is_valid_name(elf, sym) != 0;
> +}
If you call is_valid_name() directly, this function was unneeded?
> +
> +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,
I could not understand. In which case, the ELF file changes?
modpost loads the entire file to memory first..
In which scenario, the memory content changes?
> + * 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.
I could not understand this comment.
If section_index and addr both at their max values at [table_size - 2],
->table[table_size - 2].addr + 1 wraps to zero.
The table is not sorted any longer?
> + * 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;
The binary search code was implemented in a too complex way
to preserve the previous quirks.
I want to use the same comparison function for
qsort() and bsearch() to avoid paranoia.
How about this implementation?
static struct syminfo *symsearch_find_impl(struct elf_info *elf, Elf_Addr addr,
unsigned int secndx, bool
allow_negative,
Elf_Addr min_distance)
{
struct syminfo target = { .symbol_index = -1, .section_index =
secndx, .addr = addr };
struct syminfo *table = elf->symsearch->table;
unsigned int hi = elf->symsearch->table_size - 1;
unsigned int lo = 0;
struct syminfo *result = NULL;
Elf_Addr distance;
while (lo < hi) {
unsigned int mid = (lo + hi + 1) / 2;
if (syminfo_compare(&table[mid], &target) > 0)
hi = mid - 1;
else
lo = mid;
}
/*
* The target resides between lo and (lo + 1).
* If allow_negative is true, check both of them.
*/
if (allow_negative && lo + 1 < elf->symsearch->table_size &&
table[lo + 1].section_index == secndx) {
distance = table[lo + 1].addr - addr;
if (distance <= min_distance) {
min_distance = distance;
result = &table[lo + 1];
}
}
if (table[lo].section_index == secndx) {
distance = addr - table[lo].addr;
if (distance <= min_distance)
result = &table[lo];
}
return result;
}
Elf_Sym *symsearch_find_nearest(struct elf_info *elf, Elf_Addr addr,
unsigned int secndx, bool allow_negative,
Elf_Addr min_distance)
{
struct syminfo *result;
result = symsearch_find_impl(elf, addr, secndx,
allow_negative, min_distance);
if (!result)
return NULL;
return &elf->symtab_start[result->symbol_index];
}
This does not preserve the previous quirks.
If there are multiple entries with the same address,
it always returns the last element.
I did not expect sentinels.
I did not do thorough tests, but it seems to be working for me.
Also, please call symsearch_find_nearest() directly
and remove symfind_nearest_sym().
--
Best Regards
Masahiro Yamada
