Hi Dave, On 5/13/20 1:13 PM, Dave Martin wrote: > On Wed, May 13, 2020 at 12:10:25PM +0200, Michael Kerrisk (man-pages) wrote: >> Hi Dave, >> >> On 5/12/20 6:36 PM, Dave Martin wrote: >>> In reality, almost every prctl interferes with assumptions that the >>> compiler and C library / runtime rely on. prctl() can therefore >>> make userspace explode in a variety ways that are likely to be hard >>> to debug. >>> >>> This is not obvious to the uninitiated, so add a warning. >> >> Patch applied. But see my comments on patch 04. I may want to >> circle back on this patch later, since the wording feels a >> little strong to me (we simply must use prctl for some things, >> and not all of those things break user-space/runtime as far >> as I know). If you have some thoughts on softening the warning, >> let me know. > > Certainly the "if at all" can go -- this was just a suggestion > really. Yes. Gone. > Maybe the whole thing is superfluous. In C anything can screw up the > runtime if you try hard enough. I think it's at least worth alerting the reader to this issue. > The background to this patch is that things like the new > PR_PAC_RESET_KEYS and PR_SVE_SET_VL are likely to crash the program, or > place a timebomb that will explode later when someone upgrades their > toolchain or links with a new version of some library. Many existing > prctls that look equally unfriendly... > > I didn't want to say nothing at all, but I didn't want to get into the > gory details either. (Okay.) > Doing the digging to document the safety requirements of each prctl > would be a lot of work, and probably an exercise in futility anyway -- > how to use a lot of prctls safely depends on the run-time environment as > much as it does on the kernel. > > > If you want to drop this, I'm happy to add explicit notes to just the > new arm64 prctls instead for now. I just softened the warning a little; see below. Explicit notes for the new arm64 prctls would certainly be welcome. Cheers, Michael diff --git a/man2/prctl.2 b/man2/prctl.2 index 7e78fc3c1..4e2d67345 100644 --- a/man2/prctl.2 +++ b/man2/prctl.2 @@ -66,10 +66,10 @@ prctl \- operations on a process or thread manipulates various aspects of the behavior of the calling thread or process. .PP -Note that careless use of +Note that careless use of some .BR prctl () -can confuse the user-space run-time environment, -so these operations should be used with care (if at all). +operations can confuse the user-space run-time environment, +so these operations should be used with care. .PP .BR prctl () is called with a first argument describing what to do -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/
