Feedback-ID: i0fe9450f:Fastmail
Date: Thu, 13 Aug 2026 17:27:42 +0900
From: Michael Paquier <michael@paquier.xyz>
To: Andrey Rachitskiy <pl0h0yp1@gmail.com>
Cc: ilia.kashintsev@gmail.com, pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #19612: SEGV in ParseConfigFp() in guc-file.l
Message-ID: <an1__bbJ4MNfy9Lp@paquier.xyz>
References: <19612-24ccb4fc6da7786f@postgresql.org>
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On Tue, Aug 11, 2026 at 11:30:03AM +0500, Andrey Rachitskiy wrote:
> Commit 4b496a3583e already marked the YY_BUFFER_STATE local volatile
> for that longjmp path.  Commit d663f150b5e made the scanner reentrant
> and added a yyscan_t local used in the same cleanup.  That local is
> written after sigsetjmp and read after siglongjmp, but was not
> volatile, so its value is indeterminate after the jump.

Asan failure reproduced, thanks.  Your patch has missed the following
piece with yylex_init():
guc-file.l:387:17: warning: passing 'volatile yyscan_t *' (aka 'void
*volatile *') to parameter of type 'yyscan_t *' (aka 'void **')
discards qualifiers
[-Wincompatible-pointer-types-discards-qualifiers]   387 |         if
(yylex_init(&scanner) != 0)

I am wondering whether we should just use a non-volatile copy of
"scanner", just for the sake of yylex_init().  The attached seems to
work fine here with asan.

Thoughts?
--
Michael

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diff --git a/src/backend/utils/misc/guc-file.l b/src/backend/utils/misc/guc=
-file.l
index 58669a67e050..13cdba6cd7b6 100644
--- a/src/backend/utils/misc/guc-file.l
+++ b/src/backend/utils/misc/guc-file.l
@@ -354,7 +354,8 @@ ParseConfigFp(FILE *fp, const char *config_file, int de=
pth, int elevel,
 	unsigned int save_ConfigFileLineno =3D ConfigFileLineno;
 	sigjmp_buf *save_GUC_flex_fatal_jmp =3D GUC_flex_fatal_jmp;
 	sigjmp_buf	flex_fatal_jmp;
-	yyscan_t	scanner;
+	volatile yyscan_t scanner =3D NULL;
+	yyscan_t scanner_init;	/* non-volatile for yylex_init() */
 	struct yyguts_t *yyg;		/* needed for yytext macro */
 	volatile YY_BUFFER_STATE lex_buffer =3D NULL;
 	int			errorcount;
@@ -384,8 +385,9 @@ ParseConfigFp(FILE *fp, const char *config_file, int de=
pth, int elevel,
 	ConfigFileLineno =3D 1;
 	errorcount =3D 0;
=20
-	if (yylex_init(&scanner) !=3D 0)
+	if (yylex_init(&scanner_init) !=3D 0)
 		elog(elevel, "yylex_init() failed: %m");
+	scanner =3D scanner_init;
 	yyg =3D (struct yyguts_t *) scanner;
=20
 	lex_buffer =3D yy_create_buffer(fp, YY_BUF_SIZE, scanner);
@@ -559,8 +561,11 @@ parse_error:
 	}
=20
 cleanup:
-	yy_delete_buffer(lex_buffer, scanner);
-	yylex_destroy(scanner);
+	if (scanner)
+	{
+		yy_delete_buffer(lex_buffer, scanner);
+		yylex_destroy(scanner);
+	}
 	/* Each recursion level must save and restore these static variables. */
 	ConfigFileLineno =3D save_ConfigFileLineno;
 	GUC_flex_fatal_jmp =3D save_GUC_flex_fatal_jmp;

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