From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from list by lists.gnu.org with archive (Exim 4.90_1) id 1p8w1L-0002sJ-KI for mharc-grub-devel@gnu.org; Fri, 23 Dec 2022 23:20:27 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1p8w1K-0002r6-6s for grub-devel@gnu.org; Fri, 23 Dec 2022 23:20:26 -0500 Received: from mail-pj1-x102e.google.com ([2607:f8b0:4864:20::102e]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1p8w1H-0000Ya-2E for grub-devel@gnu.org; Fri, 23 Dec 2022 23:20:25 -0500 Received: by mail-pj1-x102e.google.com with SMTP id fy4so6561243pjb.0 for ; Fri, 23 Dec 2022 20:20:22 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=efficientek-com.20210112.gappssmtp.com; s=20210112; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:from:to:cc:subject:date :message-id:reply-to; bh=0M1jj4Rbcu3UTGDtPmaLjit/FEaoS+T6ydyfVTsMzNc=; b=1jxIsIA5H7cQTlPJYQ8ptXpFNHks5sOcahoRqUwDO9nnD2iLYGrdOHrPl3zL5lvmvo 10mczY3J3LjDCP4QRsrwFIzc/cecpYPTfAlWyHq2WZ70aVe8OWtpcP5iEf7ZXM6R0ETF rFgdXyMYt5t97oqYk4d9UGv03ZXqi6EzPxpQyxMzGmvwdU5eFuRSgUJBsnXIKI4W+nKB ZoUZmvPW2M3FE/BEd6xg7I+E3/Q4EK8yTaBac8n96XNkfVBBI/RRpdgnvwhBblPYPJHi AU9ZZqj0pyYpCDTdx9g76S+xtYZh0yW2ujJdadwRTaEYfAl1oyKjNHBrOwt3MBZKtjiE P0KA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=0M1jj4Rbcu3UTGDtPmaLjit/FEaoS+T6ydyfVTsMzNc=; b=QZs4ZNAMM1MY7SM0R+hYmiGiX6qKj7TaK5mq/8+LlmISIa4rAxvwtmbCfx0yoc+HGj d9phiaUK3QUiIOnGP/oUAKkiK+Y6iBQSLMir0YqMHvZoDcSeQk6o4ruty6ubKb6McDDe eVKk3yR+CMYz5LsvInKSSqmWoLr+QhkVRTmrE7QNDyhYK8CSYxbdlV/QuIvkhpcR97gJ QvtyWHAZBhtV9ZftbWP9eLQWACcna8ROyaTkD6mztNcpzhNu9f9sKIhiCpKcf+NC15M/ aiJI1ydSnviqtNWMvl7z4S/x2chNxbEkv+/Fbt/BQIWFjbl+PvAt+u0v1XNN/wNh/dZV kkKQ== X-Gm-Message-State: AFqh2kpp/lIGQ8C3xPuBEB2VZIJq/OGq9b9YlwEWGMJToS/zIp/B6jKO WYStRzeOgUnLzO9K8Z6c/Lf54Eu3qIpfm341 X-Google-Smtp-Source: AMrXdXs92mIpZAN2+JE0SNsiKnxzkUMOkytJrF5cxCQac4Km/K8zYtdlz46gxqXsHvzLj3CRVZc2kQ== X-Received: by 2002:a05:6a21:e302:b0:a3:d6:8795 with SMTP id cb2-20020a056a21e30200b000a300d68795mr15846695pzc.17.1671855620784; Fri, 23 Dec 2022 20:20:20 -0800 (PST) Received: from localhost.localdomain ([204.13.164.84]) by smtp.gmail.com with ESMTPSA id x19-20020aa79413000000b005743b5ebd7csm3324185pfo.92.2022.12.23.20.20.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 23 Dec 2022 20:20:19 -0800 (PST) From: Glenn Washburn To: grub-devel@gnu.org, Daniel Kiper Cc: Robbie Harwood , Peter Jones , Glenn Washburn Subject: [PATCH v5 11/14] gdb: Fix issue with breakpoints defined before the GRUB image is loaded Date: Fri, 23 Dec 2022 22:19:32 -0600 Message-Id: <20221224041935.787292-12-development@efficientek.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20221224041935.787292-1-development@efficientek.com> References: <20221224041935.787292-1-development@efficientek.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=2607:f8b0:4864:20::102e; envelope-from=development@efficientek.com; helo=mail-pj1-x102e.google.com X-Spam_score_int: -18 X-Spam_score: -1.9 X-Spam_bar: - X-Spam_report: (-1.9 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: grub-devel@gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: The development of GNU GRUB List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 24 Dec 2022 04:20:26 -0000 On some platforms, notably x86, software breakpoints set in GDB before the GRUB image is loaded will be cleared when the image is loaded. This is because the breakpoints work by overwriting the memory of the break- point location with a special instruction which when hit will cause the debugger to stop execution. Just before execution is resumed by the debugger, the original instruction bytes are put back. When a breakpoint is set before the GRUB image is loaded, the special debugger instruction will be written to memory and when the GRUB image is loaded by the firmware, which has no knowledge of the debugger, the debugger instruction is overwritten. To the GDB user, GDB will show the breakpoint as set, but it will never be hit. Furthermore, GDB now becomes confused, such that even deleting and re-setting the breakpoint after the GRUB image is loaded will not allow for a working breakpoint. To work around this, in run_on_start, first a watchpoint is set on _start, which will be triggered when the firmware starts loading the GRUB image. When the _start watchpoint is hit, the current breakpoints are saved to a file and then deleted by GDB before they can be overwritten by the firmware and confuse GDB. Then a temporary software breakpoint is set on _start, which will get triggered when the firmware hands off to GRUB to execute. In that breakpoint load the previously saved and deleted breakpoints now that there is no worry of them getting overwritten by the firmware. Note that watchpoints are generally types of hardware breakpoints on x86, so its deleted as soon as it gets triggered so that a minimal set of hardware breakpoints are used, allowing more for the user. Signed-off-by: Glenn Washburn --- grub-core/gdb_grub.in | 33 +++++++++++++++++++++++++++------ 1 file changed, 27 insertions(+), 6 deletions(-) diff --git a/grub-core/gdb_grub.in b/grub-core/gdb_grub.in index 281dfb5927..8e89bbf368 100644 --- a/grub-core/gdb_grub.in +++ b/grub-core/gdb_grub.in @@ -59,14 +59,35 @@ end define run_on_start # TODO: Add check to see if _start symbol is defined, if not, then # the symbols have not yet been loaded and this command will not work. - thbreak _start + watch *_start + set $break_efi_start_bpnum = $bpnum commands silent - - runtime_load_module - - if $is_user_command("onstart") - onstart + delete $break_efi_start_bpnum + + # Save the breakpoints here before the GRUB image is loaded + # into memory, then delete them. Later they will be reloaded + # once the GRUB image has been loaded. This avoids the issue + # where the loading of the GRUB image overwrites the software + # breakpoints, thus confusing GDB and effectively clearing + # those breakpoints. + save breakpoints .early-breakpoints.gdb + delete breakpoints + + tbreak _start + commands + silent + + # Reload the breakpoints now that the GRUB image has + # finished being loaded into memory. + source .early-breakpoints.gdb + + runtime_load_module + + if $is_user_command("onstart") + onstart + end + continue end continue end -- 2.34.1