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Mon, 22 May 2023 14:42:18 +0000 From: Ciaran Woodward To: "gdb-patches@sourceware.org" CC: Ciaran Woodward Subject: [PATCH] Fix scoped_value_mark not working with empty value chain Thread-Topic: [PATCH] Fix scoped_value_mark not working with empty value chain Thread-Index: AQHZjLtgRoAzhHeK2k69AlNP+iFzsw== Date: Mon, 22 May 2023 14:42:18 +0000 Message-ID: Accept-Language: en-GB, en-US Content-Language: en-GB X-MS-Has-Attach: X-MS-TNEF-Correlator: msip_labels: authentication-results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=xmos.com; x-ms-publictraffictype: Email x-ms-traffictypediagnostic: PAXPR09MB5583:EE_|VI1PR09MB4176:EE_ x-ms-office365-filtering-correlation-id: e33bebd6-fbda-4811-e9cd-08db5ad2be88 x-ms-exchange-senderadcheck: 1 x-ms-exchange-antispam-relay: 0 x-microsoft-antispam: BCL:0; x-microsoft-antispam-message-info: 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 x-forefront-antispam-report: CIP:255.255.255.255; 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However, value_mark() also returns NULL if the value chain is empty at the time of call. This lead to the situation that if the value chain was empty at the time the scoped_value_mark was created, the class would not correctly clean up the state when it was destroyed, because it believed it had already been freed. I noticed this because I was setting a watchpoint very early in my debug session, and it was becoming a software watchpoint rather than hardware. Running any command that called evaluate() beforehand (such as 'x 0') would mean that a hardware watchpoint was correctly used. After some careful examination of the differences in execution, I noticed that values were being freed later in the 'bad case', which lead me to notice the issue with scoped_value_mark. --- gdb/value.h | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) -- 2.25.1 diff --git a/gdb/value.h b/gdb/value.h index d042d816409..85a33abe7d7 100644 --- a/gdb/value.h +++ b/gdb/value.h @@ -1155,6 +1155,7 @@ class scoped_value_mark scoped_value_mark () : m_value (value_mark ()) + , m_freed (false) { } @@ -1170,16 +1171,17 @@ class scoped_value_mark /* Free the values currently on the value stack. */ void free_to_mark () { - if (m_value != NULL) + if (!m_freed) { - value_free_to_mark (m_value); - m_value = NULL; + value_free_to_mark (m_value); + m_freed = true; } } private: const struct value *m_value; + bool m_freed; }; extern struct value *value_cstring (const char *ptr, ssize_t len,