From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from list by lists.gnu.org with archive (Exim 4.90_1) id 1pFMWs-0004kw-OU for mharc-grub-devel@gnu.org; Tue, 10 Jan 2023 16:51:34 -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 1pFMWr-0004km-Hs for grub-devel@gnu.org; Tue, 10 Jan 2023 16:51:33 -0500 Received: from mail-pj1-x102a.google.com ([2607:f8b0:4864:20::102a]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1pFMWp-0002Sw-Rt for grub-devel@gnu.org; Tue, 10 Jan 2023 16:51:33 -0500 Received: by mail-pj1-x102a.google.com with SMTP id c8-20020a17090a4d0800b00225c3614161so17905652pjg.5 for ; Tue, 10 Jan 2023 13:51:31 -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=myRB2bvnMWNgNz/5uJZ31ZW1ZstENmOPUmpRIZCJQ4PWGgvVMY03whUdRQ0JTdjV1W d4n+4gBkHVyBE4uvEXIJXAmksCrOt7315/HisPkw6X10uxPeofIbhu3pMRLcV0kbNoo/ ljY/1yJ3qrsffbQsxi2pr5FnUQz4DeDMzVFWEluivYrBVn/MpukkesKBX5eo5Q4HgZbx GW3keI5M8nqQtFi2qLMJ6IFM6sgb93Q5qhYkGaeBpVOvGGCitBfL4N6Hn3XIoVzUSLwi liv0XgsWkwioixUwEdQZd7ChzhTTZCTc3c2eD6n/E3hnQthM8bRmNsqcZxQ0qU67FRmq xIAg== 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=vLLqXUmPhiNT72qTkynp+Sfjrr2uuZTV2iU0R1lTnD88r+h3W+rhawbSPzjIiL9Rog bwnXfJkHdyRym0k2Vx6pJEXvYPbSOuok4wWwvqCLV+OlSd7Re+Z81+Ye7w+hF2H5x3gy Y+XWNd4LhpbGq4EcFeAVa/J+KHtZPzYtYfE6Raj3Ch/S+uMbqV0DO7U0jU2m2l6nVy3s 96PqLinvC8GhD/T6EzjVYSNkzSlfyyIYP605pVzVI/HEkBvLFRkRDXPYKLoAvXHrmd7y z/xDZ29Y2IIlU8Z5DnUrGQv2ANWCOmYsh1WpFc24MyjZ0/sUMKTCWHDkPBty6Gg3RCXy C5Rw== X-Gm-Message-State: AFqh2krd/voyU4GP+GgxoyvpayOEBkQTlOSQdFu7u5rGzgWIqMaxwG7X Pbguf/1R4aIi4/5cDF/+OnkiZkajyxYqOkc1 X-Google-Smtp-Source: AMrXdXtUXVO8j0CQSANhj/6gohfzf/IYO5YSkSe1HkPzoZaotqHb9Q9PH7ktXJfwT22CDD+EUDw3ZA== X-Received: by 2002:a17:902:aa45:b0:192:86ff:c26 with SMTP id c5-20020a170902aa4500b0019286ff0c26mr262627plr.66.1673387489899; Tue, 10 Jan 2023 13:51:29 -0800 (PST) Received: from crass-HP-ZBook-15-G2.lan ([199.254.238.56]) by smtp.gmail.com with ESMTPSA id u15-20020a1709026e0f00b00186c3af9644sm7760798plk.273.2023.01.10.13.51.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 10 Jan 2023 13:51:29 -0800 (PST) From: Glenn Washburn To: grub-devel@gnu.org, Daniel Kiper Cc: Robbie Harwood , Peter Jones , Glenn Washburn Subject: [PATCH v6 11/14] gdb: Fix issue with breakpoints defined before the GRUB image is loaded Date: Tue, 10 Jan 2023 15:50:38 -0600 Message-Id: <20230110215041.1247699-12-development@efficientek.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20230110215041.1247699-1-development@efficientek.com> References: <20230110215041.1247699-1-development@efficientek.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=2607:f8b0:4864:20::102a; envelope-from=development@efficientek.com; helo=mail-pj1-x102a.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: Tue, 10 Jan 2023 21:51:33 -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