[PATCH] grep: fall back to interpreter mode if JIT fails

From: Mathias Krause <minipli@grsecurity.net>
Date: Dec 16, 2022, 12:15 UTC
Message-ID: <20221216121557.30714-1-minipli@grsecurity.net>

From: Carlo Marcelo Arenas Belón <carenas@gmail.com>

Under Linux systems with SELinux's 'deny_execmem' or PaX's MPROTECT enabled, the allocation of PCRE2's JIT rwx memory may be prohibited, making pcre2_jit_compile() fail with PCRE2_ERROR_NOMEMORY (-48):

  [user@fedora git]$ git grep -c PCRE2_JIT
  grep.c:1

  [user@fedora git]$ # Enable SELinux's W^X policy
  [user@fedora git]$ sudo semanage boolean -m -1 deny_execmem

  [user@fedora git]$ # JIT memory allocation fails, breaking 'git grep'
  [user@fedora git]$ git grep -c PCRE2_JIT
  fatal: Couldn't JIT the PCRE2 pattern 'PCRE2_JIT', got '-48'

Instead of failing hard in this case and making 'git grep' unusable on such systems, simply fall back to interpreter mode, leading to a much better user experience.

Such a change was already proposed 4 years ago [1] but wasn't merged for unknown reasons.

1. https://lore.kernel.org/r/20181209230024.43444-3-carenas@gmail.com

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Mathias Krause <minipli@grsecurity.net>	# tweaked changelog, added comment
---
 grep.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/grep.c b/grep.c
index 06eed694936c..f2ada528b21d 100644
--- a/grep.c
+++ b/grep.c
@@ -317,8 +317,21 @@ static void compile_pcre2_pattern(struct grep_pat *p, const struct grep_opt *opt
 	pcre2_config(PCRE2_CONFIG_JIT, &p->pcre2_jit_on);
 	if (p->pcre2_jit_on) {
 		jitret = pcre2_jit_compile(p->pcre2_pattern, PCRE2_JIT_COMPLETE);
-		if (jitret)
-			die("Couldn't JIT the PCRE2 pattern '%s', got '%d'\n", p->pattern, jitret);
+		if (jitret) {
+			/*
+			 * Even though pcre2_config(PCRE2_CONFIG_JIT, ...)
+			 * indicated JIT support, the library might still
+			 * fail to generate JIT code for various reasons,
+			 * e.g. when SELinux's 'deny_execmem' or PaX's
+			 * MPROTECT prevent creating W|X memory mappings.
+			 *
+			 * Instead of faling hard, fall back to interpreter
+			 * mode, just as if the pattern was prefixed with
+			 * '(*NO_JIT)'.
+			 */
+			p->pcre2_jit_on = 0;
+			return;
+		}
 
 		/*
 		 * The pcre2_config(PCRE2_CONFIG_JIT, ...) call just

-- 
2.35.1