On 1/6/21 10:17 AM, Adhemerval Zanella wrote:
__libc_use_alloca/alloca is replaced with malloc regardless.
These allocations are so small that they should be put on the stack
instead of using malloc. I did that in Gnulib by installing the attached
patch. The idea is that the resulting regexec.c file should be copyable
unchanged into glibc.
From a Gnulib point of view this code uses a 20 KiB frame (on a 64-bit
host) which goes past the usual 4032-byte limit for stack frames, but I
think we can stretch the point here. In glibc the limit is 64 KiB so
there's no problem.
From 025e89118912adaa309374201c9012f6fb46d583 Mon Sep 17 00:00:00 2001
From: Paul Eggert
Date: Fri, 8 Jan 2021 14:22:15 -0800
Subject: [PATCH] regexec: remove alloca usage in build_trtable
MIME-Version: 1.0
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Prompted by this different change proposed by Adhemerval Zanella:
https://sourceware.org/pipermail/libc-alpha/2021-January/121373.html
* lib/regexec.c (build_trtable): Prevent inlining,
so that it doesn’t bloat the caller’s stack.
Use auto variables instead of alloca/malloc.
After these changes, build_trtable’s total stack allocation is
only 20 KiB on a 64-bit machine, and this is less than glibc’s 64
KiB cutoff so there’s little point to using alloca to shrink it.
Although Gnulib traditionally has used a 4 KiB cutoff, going to 20
KiB here should not be a significant problem in practice;
Gnulib-using packages concerned about overflow of tiny stacks can
compile with something like gcc -fstack-clash-protection.
* config/srclist.txt: Comment out regexec.c for now.
---
ChangeLog | 14 +
config/srclist.txt | 2 +-
lib/regexec.c | 75 --
3 files changed, 28 insertions(+), 63 deletions(-)
diff --git a/ChangeLog b/ChangeLog
index 540f15a3c..708a266b0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,19 @@
2021-01-08 Paul Eggert
+ regexec: remove alloca usage in build_trtable
+ Prompted by this different change proposed by Adhemerval Zanella:
+ https://sourceware.org/pipermail/libc-alpha/2021-January/121373.html
+ * lib/regexec.c (build_trtable): Prevent inlining,
+ so that it doesn’t bloat the caller’s stack.
+ Use auto variables instead of alloca/malloc.
+ After these changes, build_trtable’s total stack allocation is
+ only 20 KiB on a 64-bit machine, and this is less than glibc’s 64
+ KiB cutoff so there’s little point to using alloca to shrink it.
+ Although Gnulib traditionally has used a 4 KiB cutoff, going to 20
+ KiB here should not be a significant problem in practice;
+ Gnulib-using packages concerned about overflow of tiny stacks can
+ compile with something like gcc -fstack-clash-protection.
+
scratch_buffer: add scratch_buffer_dupfree macro
* lib/scratch_buffer.h (__libc_scratch_buffer_dupfree):
New macro, needed to support recent changes in this module.
diff --git a/config/srclist.txt b/config/srclist.txt
index e7ceeb088..d669fd8f0 100644
--- a/config/srclist.txt
+++ b/config/srclist.txt
@@ -69,7 +69,7 @@ $LIBCSRC posix/regex.c lib
#$LIBCSRC posix/regex.h lib
#$LIBCSRC posix/regex_internal.c lib
#$LIBCSRC posix/regex_internal.h lib
-$LIBCSRC posix/regexec.c lib
+#$LIBCSRC posix/regexec.c lib
#$LIBCSRC stdlib/canonicalize lib/canonicalize-lgpl.c
#$LIBCSRC sysdeps/generic/eloop-threshold.h lib
$LIBCSRC time/timegm.c lib
diff --git a/lib/regexec.c b/lib/regexec.c
index 889b10be9..f7b4f9cfc 100644
--- a/lib/regexec.c
+++ b/lib/regexec.c
@@ -3247,7 +3247,7 @@ expand_bkref_cache (re_match_context_t *mctx, re_node_set *cur_nodes,
/* Build transition table for the state.
Return true if successful. */
-static bool
+static bool __attribute_noinline__
build_trtable (const re_dfa_t *dfa, re_dfastate_t *state)
{
reg_errcode_t err;
@@ -3255,36 +3255,20 @@ build_trtable (const re_dfa_t *dfa, re_dfastate_t *state)
int ch;
bool need_word_trtable = false;
bitset_word_t elem, mask;
- bool dests_node_malloced = false;
- bool dest_states_malloced = false;
Idx ndests; /* Number of the destination states from 'state'. */
re_dfastate_t **trtable;
- re_dfastate_t **dest_states = NULL, **dest_states_word, **dest_states_nl;
- re_node_set follows, *dests_node;
- bitset_t *dests_ch;
+ re_dfastate_t *dest_states[SBC_MAX];
+ re_dfastate_t *dest_states_word[SBC_MAX];
+ re_dfastate_t *dest_states_nl[SBC_MAX];
+ re_node_set follows;
bitset_t acceptable;
- struct dests_alloc
- {
-re_node_set dests_node[SBC_MAX];
-bitset_t dests_ch[SBC_MAX];
- } *dests_alloc;
-
/* We build DFA states which corresponds to the destination nodes
from 'state'. 'dests_node[i]' represents the nodes which i-th
destination state contains, and 'dests_ch[i]' represents the
characters which i-th destination state accepts. */
- if (__libc_use_alloca (sizeof (struct dests_alloc)))
-dests_alloc = (struct dests_alloc *) alloca
