Date: Fri, 21 Nov 2014 13:49:35 +0000 From: David Drysdale <drysdale@...gle.com> To: Christoph Hellwig <hch@...radead.org> Cc: Rich Felker <dalias@...ifal.cx>, libc-alpha <libc-alpha@...rceware.org>, Andrew Morton <akpm@...ux-foundation.org>, Linux API <linux-api@...r.kernel.org>, Andy Lutomirski <luto@...capital.net>, musl@...ts.openwall.com Subject: Re: [RFC] Possible new execveat(2) Linux syscall On Fri, Nov 21, 2014 at 10:13 AM, Christoph Hellwig <hch@...radead.org> wrote: > On Sun, Nov 16, 2014 at 02:52:46PM -0500, Rich Felker wrote: >> I've been following the discussions so far and everything looks mostly >> okay. There are still issues to be resolved with the different >> semantics between Linux O_PATH and what POSIX requires for O_EXEC (and >> O_SEARCH) but as long as the intent is that, once O_EXEC is defined to >> save the permissions at the time of open and cause them to be used in >> place of the current file permissions at the time of execveat > > As far as I can tell we only need the little patch below to make Linux > O_PATH a valid O_SEARCH implementation. Rich, you said you wanted to > look over it? > > For O_EXEC my interpretation is that we basically just need this new > execveat syscall + a patch to add FMODE_EXEC and enforce it. So we > wouldn't even need the O_PATH|3 hack. But unless someone more familar > with the arcane details of the Posix language verifies it I'm tempted to > give up trying to help to implent these flags :( I'm not particularly familiar with POSIX details either, but I thought the O_PATH|3 hack would be needed for the interaction with O_ACCMODE -- just using FMODE_EXEC as O_EXEC would confuse existing code that examines (flags & O_ACCMODE). >From : "Applications shall specify exactly one of the ...five ... file access modes ... O_EXEC / O_RDONLY / O_RDWR / O_SEARCH / O_WRONLY" (and O_EXEC and O_SEARCH are allowed to be the same value, as one only applies to files and the other only applies to directories). As O_ACCMODE is 3, there are only 4 possible access modes that work with any existing code that checks (flags & O_ACCMODE), and 3 of the values are taken (0=O_RDONLY, 1=O_WRONLY, 2=O_RDWR). So I guess that's where the idea for the |3 hack comes from.  http://pubs.opengroup.org/onlinepubs/9699919799/functions/open.html
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