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Thu, 22 Apr 2021 15:46:29 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 43017AD8A; Thu, 22 Apr 2021 15:46:28 +0000 (UTC) X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" X-Inumbo-ID: f42098b1-f2d4-43d1-beb9-7b1e644c3192 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1619106388; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=hldtQOqJyVc74yWwVP2TFRdnx9LVlWLkWySq+1YHgcg=; b=vTrA2M1dT+rJ7ugE/ZpyOBm11XFyI3H9wAOCs19oOzeY+W5kOqAmL/yDFAtxajU1yL3jaS LPg/cgp1r4OZ5CEZB/CPKHp1aCjIQ3Ql+Fgm00bQQSiBCLgvUOB7gBXJqXNetjEE6wn839 qinFcXDxyM7kj33PpaoauDFmDUjZ+SE= Subject: Re: [PATCH 7/8] x86/EFI: keep debug info in xen.efi To: =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= Cc: "xen-devel@lists.xenproject.org" , Andrew Cooper , Wei Liu References: <96cb4039-5f43-8eac-d855-c9f1587ed2d0@suse.com> <6023c1d3-4f50-d207-1ea1-30dd1d5f68d2@suse.com> <3cda296d-e88e-5fe0-4924-4cf974c4f909@suse.com> From: Jan Beulich Message-ID: <8fabc56b-8e2d-96cd-e9b2-81bf38777d52@suse.com> Date: Thu, 22 Apr 2021 17:46:28 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit On 22.04.2021 16:56, Roger Pau Monné wrote: > On Thu, Apr 22, 2021 at 01:03:13PM +0200, Jan Beulich wrote: >> On 22.04.2021 10:14, Roger Pau Monné wrote: >>> On Wed, Apr 21, 2021 at 05:38:42PM +0200, Jan Beulich wrote: >>>> On 21.04.2021 17:30, Roger Pau Monné wrote: >>>>> On Wed, Apr 21, 2021 at 03:06:36PM +0200, Jan Beulich wrote: >>>>>> On 21.04.2021 13:15, Roger Pau Monné wrote: >>>>>>> On Thu, Apr 01, 2021 at 11:47:03AM +0200, Jan Beulich wrote: >>>>>>>> --- a/xen/arch/x86/xen.lds.S >>>>>>>> +++ b/xen/arch/x86/xen.lds.S >>>>>>>> @@ -312,10 +312,60 @@ SECTIONS >>>>>>>> *(.reloc) >>>>>>>> __base_relocs_end = .; >>>>>>>> } >>>>>>>> - /* Trick the linker into setting the image size to exactly 16Mb. */ >>>>>>>> - . = ALIGN(__section_alignment__); >>>>>>>> - DECL_SECTION(.pad) { >>>>>>>> - . = ALIGN(MB(16)); >>>>>>>> + .debug_abbrev ALIGN(1) (NOLOAD) : { >>>>>>>> + *(.debug_abbrev) >>>>>>>> + } >>>>>>>> + .debug_info ALIGN(1) (NOLOAD) : { >>>>>>>> + *(.debug_info) >>>>>>>> + *(.gnu.linkonce.wi.*) >>>>>>>> + } >>>>>>>> + .debug_types ALIGN(1) (NOLOAD) : { >>>>>>>> + *(.debug_types) >>>>>>>> + } >>>>>>>> + .debug_str ALIGN(1) (NOLOAD) : { >>>>>>>> + *(.debug_str) >>>>>>>> + } >>>>>>>> + .debug_line ALIGN(1) (NOLOAD) : { >>>>>>>> + *(.debug_line) >>>>>>>> + *(.debug_line.*) >>>>>>>> + } >>>>>>>> + .debug_line_str ALIGN(1) (NOLOAD) : { >>>>>>>> + *(.debug_line_str) >>>>>>>> + } >>>>>>>> + .debug_names ALIGN(4) (NOLOAD) : { >>>>>>>> + *(.debug_names) >>>>>>>> + } >>>>>>>> + .debug_frame ALIGN(4) (NOLOAD) : { >>>>>>>> + *(.debug_frame) >>>>>>>> + } >>>>>>>> + .debug_loc ALIGN(1) (NOLOAD) : { >>>>>>>> + *(.debug_loc) >>>>>>>> + } >>>>>>>> + .debug_loclists ALIGN(4) (NOLOAD) : { >>>>>>>> + *(.debug_loclists) >>>>>>>> + } >>>>>>>> + .debug_ranges ALIGN(8) (NOLOAD) : { >>>>>>>> + *(.debug_ranges) >>>>>>>> + } >>>>>>>> + .debug_rnglists ALIGN(4) (NOLOAD) : { >>>>>>>> + *(.debug_rnglists) >>>>>>>> + } >>>>>>>> + .debug_addr ALIGN(8) (NOLOAD) : { >>>>>>>> + *(.debug_addr) >>>>>>>> + } >>>>>>>> + .debug_aranges ALIGN(1) (NOLOAD) : { >>>>>>>> + *(.debug_aranges) >>>>>>>> + } >>>>>>>> + .debug_pubnames ALIGN(1) (NOLOAD) : { >>>>>>>> + *(.debug_pubnames) >>>>>>>> + } >>>>>>>> + .debug_pubtypes ALIGN(1) (NOLOAD) : { >>>>>>>> + *(.debug_pubtypes) >>>>>>>> + } >>>>>>>> + /* Trick the linker into setting the image size to no less than 16Mb. */ >>>>>>>> + __image_end__ = .; >>>>>>>> + .pad ALIGN(__section_alignment__) : { >>>>>>>> + . = __image_end__ < __image_base__ + MB(16) ? ALIGN(MB(16)) : .; >>>>>>> >>>>>>> I think this is inside an ifdef EFI region, since this is DWARF info >>>>>>> couldn't it also be present when building with EFI disabled? >>>>>> >>>>>> Of course (and it's not just "could" but "will"), yet the linker will >>>>>> do fine (and perhaps even better) without when building ELF. Also >>>>>> note that we'll be responsible for keeping the list of sections up-to- >>>>>> date. The linker will recognize Dwarf sections by looking for a >>>>>> .debug_ prefix. We can't use such here (or at least I'm not aware of >>>>>> a suitable mechanism); .debug_* would mean munging together all the >>>>>> different kinds of Dwarf sections. Hence by limiting the explicit >>>>>> enumeration to PE, I'm trying to avoid anomalies in ELF down the road. >>>>> >>>>> Right, so we will have to keep this list of debug_ sections updated >>>>> manually if/when more of those appear as part of DWARF updates? >>>> >>>> Yes. >>>> >>>>> Do we have a way to get some kind of warning or error when a new >>>>> section not explicitly handled here appears? >>>> >>>> ld 2.37 will start warning about such sections, as they'd land at >>>> VA 0 and hence below image base. >>> >>> That seems like a bug in ld? >>> >>> The '--image-base' option description mentions: "This is the lowest >>> memory location that will be used when your program or dll is >>> loaded.", so I would expect that if the option is used the default VA >>> should be >= image-base, or else the description of the option is not >>> consistent, as ld will still place sections at addresses below >>> image-base. >> >> ld's "general" logic is pretty ELF-centric. Hence debugging sections >> get placed at VA 0 by default, not matter what the (PE-specific) >> --image-base says. Whether that's a bug though I'm not sure: There >> are no really good alternatives that could be used by default. Doing >> what we do here (and what e.g. Cygwin does) via linker script may not >> be appropriate in the common case. In particular it is not generally >> correct for debug info to be part of what gets loaded into memory. > > So with this change here you placate the warnings from newer ld about > having a VA < image base, It's not just about silencing the warnings. The resulting image is unusable when the sections don't get placed at a suitable VA. > but the end result is that now the debug > sections will also get loaded when booted from the EFI loader? > (because the NOLOAD doesn't have any effect with PE) Yes. I currently see no other way to retain debug info in xen.efi. But to be clear, the memory debug info occupies isn't lost - we still free space from _end (or alike) onwards. .reloc, for example, also lives there. And I was wondering whether we shouldn't keep .comment this way as well. Jan