Hal Finkel via llvm-dev
2016-Oct-05 17:44 UTC
[llvm-dev] Using '__attribute__((section("name")))' for inline assembly injection
Would it be useful for Clang to warn about section names with unusual characters? -Hal ----- Original Message -----> From: "Reid Kleckner via llvm-dev" <llvm-dev at lists.llvm.org> > To: "Martin J. O'Riordan" <martin.oriordan at movidius.com> > Cc: "LLVM Developers" <llvm-dev at lists.llvm.org> > Sent: Wednesday, October 5, 2016 12:06:51 PM > Subject: Re: [llvm-dev] Using '__attribute__((section("name")))' for inline assembly injection > > > > People also try to use > __attribute__((section(".mysection,\"rwx\", at progbits"))) to get > executable code sections for toy experiments in self-modifying code, > but we don't support that either. Basically, we take the section > name and always quote it, unlike GCC, and I don't think we want to > change that. Normally LLVM doesn't round trip through assembly, so > it would be infeasible to attempt to interpret strings in section > names in all the ways that GCC allows through to gas. > > > My advice for users in this situation would be to try the naked > attribute again, since we've fixed several bugs around it in the > last two years. If that doesn't work, adding a standalone assembly > file to the project has always been and will continue to be the most > reliable way to implement a function with a custom prologue. > > > On Wed, Oct 5, 2016 at 1:56 AM, Martin J. O'Riordan via llvm-dev < > llvm-dev at lists.llvm.org > wrote: > > > > > > > I recently examined a bug in a program, and it turned out that the > customer was using the section attribute as a form of > inline-assembly mechanism, with something like: > > > > __attribute__((section(“sectionName\n asm \n asm \n asm ”))) > > > > this was really ugly and not at all obvious where the problem > originated. Is there any way of getting LLVM or CLang to validate > the name used in the section attribute? The GCC definition says that > this only permits alpha-numeric characters, but I know that it is > also very common for people to use ‘ _ ’ and ‘ . ’ in section names > so alpha-numeric checking is probably a bit over-zealous, but > embedded newlines definitely should be considered suspicious. Or > perhaps there is an existing check in ‘ clang-tidy ’? > > > > The reason they were doing this in the first place, is that the ‘ > naked ’ attribute does not do what they want, which was to allow > them to build custom prologue and epilogue code for a function. > > > > Thanks, > > > > MartinO > > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev > > > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >-- Hal Finkel Lead, Compiler Technology and Programming Languages Leadership Computing Facility Argonne National Laboratory
Tobias Edler von Koch via llvm-dev
2016-Oct-05 20:29 UTC
[llvm-dev] Using '__attribute__((section("name")))' for inline assembly injection
On 10/05/2016 12:44 PM, Hal Finkel via llvm-dev wrote:> Would it be useful for Clang to warn about section names with unusual > characters?It's tricky to define 'unusual' though. I'd prefer warnings for newline characters only, to be safe. We have customers embedding the file name in section names... on Windows this gets you lovely names like ".bla.D:\Some\Path\xyz.cpp", which are unusual but perfectly fine. Tobias -- Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20161005/79a370c9/attachment.html>
Joerg Sonnenberger via llvm-dev
2016-Oct-05 20:39 UTC
[llvm-dev] Using '__attribute__((section("name")))' for inline assembly injection
On Wed, Oct 05, 2016 at 12:44:29PM -0500, Hal Finkel via llvm-dev wrote:> Would it be useful for Clang to warn about section names with unusual characters?I don't think it is common enough and you can always check the output easily with readelf/objdump. Joerg
mats petersson via llvm-dev
2016-Oct-05 20:48 UTC
[llvm-dev] Using '__attribute__((section("name")))' for inline assembly injection
On 5 October 2016 at 21:39, Joerg Sonnenberger via llvm-dev < llvm-dev at lists.llvm.org> wrote:> On Wed, Oct 05, 2016 at 12:44:29PM -0500, Hal Finkel via llvm-dev wrote: > > Would it be useful for Clang to warn about section names with unusual > characters? > > I don't think it is common enough and you can always check the output > easily with readelf/objdump. >I would think that it's very uncommon, however, it is also terribly difficult to detect, and I'd argue that it's unlikely that someone actually wishes to have newlines or other "non-printable"/"control" characters in the section name. And if it's a warning that is enabled by default but possible to turn off with "-wno-weird-sectionname", I'd say it would be little harm - and not a huge maintenance burden. Knowing to check that the section names contain weirdness or otherwise debug "why the heck doesn't this code work in Clang, when it compiels without warning, and is fine when compiled in gcc or whatever" is really not at all easy. I do realise that "every warning and such is a maintenance burden", but although I have not encountered this problem, I'd definitely prefer a warning than having to figure what went wrong... -- Mats> > Joerg > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20161005/086c0126/attachment.html>