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a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=xqWC_Br6kY4A:10 a=IkcTkHD0fZMA:10 a=x6icFKpwvdMA:10 a=GoEa3M9JfhUA:10 a=VkNPw1HP01LnGYTKEx00:22 a=vTr9H3xdAAAA:8 a=JfrnYn6hAAAA:8 a=2UGR-1-_37yoZpKuNr4A:9 a=QEXdDO2ut3YA:10 a=1CNFftbPRP8L7MoqJWF3:22 cc=ntf awl=host:12124 X-Proofpoint-GUID: LI-MfO1c5y_PjCKjCNUv-_WHUvIfollg X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUxMDMwMDEyNiBTYWx0ZWRfX1B+M26hDyc6R pvnBG3SXLHtnnZLURog4eBAowVB+rU/jisJmOcVzT64a+ws2l9jLOFPnyY8cRM2HSvW9Qopjbud LkXT77+doUu4whCPCMSwYCnNa+8Op2nJpSoZulMBEzsJ2CJzXSDEau9X12e8m0rlhqBYivu19HD OPg62u1DcsRGnLDG/a4JIy+mey8T9T0+R7oPwdgNpaNzUD99OKGaYdP8K5mNKbhS99ZrqZOkqhh S0cfOa67Rri1CE2+0vME+jaOAD8L/Cg+qPJD8zV46qEtd8a1oL91vzCLYflb6BzCqELu16ZMVt3 QP+iVgAnzxXDxAl326kdS08taUFBfxVT19BbyLEOimYOrkyW94EIvRKYuI+g1PpjFvRxK860Uds dO8V8TfGPTo/nfpzPphTdqkMSjizRwZXo9MeEsv71xuYMwrTSQ0= X-Proofpoint-ORIG-GUID: LI-MfO1c5y_PjCKjCNUv-_WHUvIfollg > On Thu, Oct 30, 2025 at 3:26=E2=80=AFAM Peter Zijlstra wrote: >> >> On Wed, Oct 29, 2025 at 11:53:32PM -0700, Fangrui Song wrote: >> > I've been following the SFrame discussion and wanted to share some >> > concerns about its viability for userspace adoption, based on concrete >> > measurements and comparison with existing compact unwind >> > implementations in LLVM. >> > >> > **Size overhead concerns** >> > >> > Measurements on a x86-64 clang binary show that .sframe (8.87 MiB) is >> > approximately 10% larger than the combined size of .eh_frame and >> > .eh_frame_hdr (8.06 MiB total). This is problematic because .eh_frame >> > cannot be eliminated - it contains essential information for restoring >> > callee-saved registers, LSDA, and personality information needed for >> > debugging (e.g. reading local variables in a coredump) and C++ >> > exception handling. >> > >> > This means adopting SFrame would result in carrying both formats, with >> > a large net size increase. >> >> So the SFrame unwinder is fairly simple code, but what does an .eh_frame >> unwinder look like? Having read most of the links in your email, there >> seem to be references to DWARF byte code interpreters and stuff like >> that. >> >> So while the format compactness is one aspect, the thing I find no >> mention of, is the unwinder complexity. >> >> There have been a number of attempts to do DWARF unwinding in >> kernel space and while I think some architecture do it, x86_64 has had >> very bad experiences with it. At some point I think Linus just said no >> more, no DWARF, not ever. >> >> So from a situation where compilers were generating bad CFI unwind >> information, a horribly complex unwinder that could crash the kernel >> harder than the thing it was reporting on and manual CFI annotations in >> assembly that were never quite right, objtool and ORC were born. >> >> The win was many: >> >> - simple robust unwinder >> - no manual CFI annotations that could be wrong >> - no reliance on compilers that would get it wrong >> >> and I think this is where SFrame came from. I don't think the x86_64 >> Linux kernel will ever natively adopt SFrame, ORC works really well for >> us. >> >> However, we do need something to unwind userspace. And yes, personally >> I'm in the frame-pointer camp, that's always worked well for me. >> Distro's however don't seem to like it much, which means that every time >> I do have to profile something userspace, I get to rebuild all the >> relevant code with framepointers on (which is not hard, but tedious). >> >> Barring that, we need something for which the unwind code is simple and >> robust -- and I *think* this has disqualified .eh_frame and full on >> DWARF. >> >> And this is again where SFrame comes in -- its unwinder is simple, >> something we can run in kernel space. >> >> I really don't much care for the particulars, and frame pointers work >> for me -- but I do care about the kernel unwinder code. It had better be >> simple and robvst. >> >> So if you want us to use .eh_frame, great, show us a simple and robust >> unwinder. > > Hi Peter, > > Thanks for this perspective=E2=80=94the unwinder complexity concern is > absolutely valid and critical for kernel use. > To clarify my motivation: I've seen attempts to use SFrame for > userspace adoption > (https://fedoraproject.org/wiki/Changes/SFrameInBinaries ), and I > believe it's not viable for that purpose given the size overhead I > documented. My concerns are primarily about userspace adoption, not > the kernel's internal unwinding. > > If SFrame is exclusively a kernel-space feature, it could be > implemented entirely within objtool =E2=80=93 similar to how objtool --li= nk > --orc generates ORC info for vmlinux.o. This approach would eliminate > the need for any modifications to assemblers and linkers, while > allowing SFrame to evolve in any incompatible way. > > For userspace, we could instead modify assemblers and linkers to > support a more compact format or an extension to .eh_frame , but it > won't be SFrame (all of Apple=E2=80=99s compact unwind, ARM EHABI=E2=80= =99s > exidx/extab, and Microsoft=E2=80=99s pdata/xdata can implement C++ except= ion > handling , while SFrame can't, leading to a huge missed opportunity.) The purpose of SFrame is not to be a more compact replacement for .eh_frame. It is intended to be used to walk stacks, not to unwind them.