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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 02 Jun 2025 14:10:50.8627 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 62579cd2-e5cd-4e98-7fde-08dda1df47b8 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: AM2PEPF0001C716.eurprd05.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: AS8PR08MB6279 Hi everyone, A gentle ping :) On Mon, May 19, 2025 at 03:06:29PM +0100, Yury Khrustalev wrote: > Hi, > > I'm working on an RFC patch for Glibc to make use of the newly added > shadow_stack_token field in struct clone_args in [1] on arm64 targets. > > I encountered the following problem. Glibc might be built with newer > version of struct clone_args than the currently running kernel. In > this case, we may attempt to use a non-zero value in the new field > in args (and pass size bigger than expected by the kernel) and the > kernel will reject the syscall with E2BIG error. > > This seems to be due to a fail-early approach. The unexpected non- > zero values beyond what's supported by the kernel may indicate that > userspace expects something to happen (and may even have allocated > some resources). So it's better to indicate a problem rather than > silently ignore this and have userspace encounter an error later. > > However, it creates difficulty with using extended "versions" of > the clone3 syscall. AFAIK, there is no way to ask kernel about > the supported size of struct clone_args except for making syscalls > with decreasing value of size until we stop getting E2BIG. > > This seems fragile and may call for writing cumbersome code. In essence, > we will have to have clone30(), clone31(), clone32()... wrappers which > probably defeats the point of why clone3 was added: > > > if (clone32_supported && clone32(...) == -1 && errno == E2BIG) > { > clone32_supported = false; > /* ... */ > } > else if (clone31_supported && clone31(...) == -1 && errno == E2BIG) > { > clone12_supported = false; > /* ... */ > } > ... > > Is there a neat way to work around this? What was the idea for extending > clone_args in practice? > > I suppose we can't rely on kernel version because support for extended > clone_args can be backported. In any case, we'd have to do a syscall > for this (it would probably be great to have kernel version in auxv). > > I appreciate any advice here. > > Thanks, > Yury > > > [1]: https://lore.kernel.org/all/20250416-clone3-shadow-stack-v16-0-2ffc9ca3917b@kernel.org/ > Kind regards, Yury