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([2a01:cb14:58d:8400:ecf6:58e2:9c06:a308]) by smtp.gmail.com with ESMTPSA id l185sm2742164wml.44.2020.04.01.06.43.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 01 Apr 2020 06:43:23 -0700 (PDT) Subject: Re: [PATCH v2 04/10] objtool: check: Ignore empty alternative groups To: Miroslav Benes Cc: linux-kernel@vger.kernel.org, jpoimboe@redhat.com, peterz@infradead.org, raphael.gault@arm.com References: <20200327152847.15294-1-jthierry@redhat.com> <20200327152847.15294-5-jthierry@redhat.com> From: Julien Thierry Message-ID: <1f53ee68-3bcc-7de9-beb9-df812b2e3613@redhat.com> Date: Wed, 1 Apr 2020 14:43:22 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=windows-1252; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 4/1/20 1:53 PM, Miroslav Benes wrote: > On Fri, 27 Mar 2020, Julien Thierry wrote: > >> Atlernative section can contain entries for alternatives with no >> instructions. Objtool will currently crash when handling such an entry. >> >> Just skip that entry, but still give a warning to discourage useless >> entries. >> >> Signed-off-by: Julien Thierry >> --- >> tools/objtool/check.c | 6 ++++++ >> 1 file changed, 6 insertions(+) >> >> diff --git a/tools/objtool/check.c b/tools/objtool/check.c >> index 74353b2c39ce..5c03460f1f07 100644 >> --- a/tools/objtool/check.c >> +++ b/tools/objtool/check.c >> @@ -904,6 +904,12 @@ static int add_special_section_alts(struct objtool_file *file) >> } >> >> if (special_alt->group) { >> + if (!special_alt->orig_len) { >> + WARN_FUNC("empty alternative entry", >> + orig_insn->sec, orig_insn->offset); >> + continue; >> + } >> + >> ret = handle_group_alt(file, special_alt, orig_insn, >> &new_insn); >> if (ret) > > Probably the first time I am looking at alternatives handling in objtool, > so I must be missing something, but is this even possible now? I mean > get_alt_entry() in special.c sets alt->orig_len when alt->group is true > (which means .alternatives section) to something which cannot be zero. > What I see is: if (alt->group) { alt->orig_len = *(unsigned char *)(sec->data->d_buf + offset + entry->orig_len); alt->new_len = *(unsigned char *)(sec->data->d_buf + offset + entry->new_len); } And as far as I can tell, "alt->orig_len" can be 0 if the entry in the .altinstructions section of the .o file has the length set to 0. I don't know how the alternative section generation works on x86, but on arm64 it's just a computed assembly offset which can be 0. > Is this a preparatory patch for arm64, where this could happen? If yes, it > would be better to mention it in the changelog. > It used to happen on arm64, but the fix [1] was picked. I can add that link to the commit if necessary. [1] https://lkml.org/lkml/2020/1/9/708 Cheers, -- Julien Thierry