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Message-ID: <20260421131308.GN1827@brightrain.aerifal.cx>
Date: Tue, 21 Apr 2026 09:13:17 -0400
From: Rich Felker <dalias@...c.org>
To: Florian Schmaus <florian.schmaus@...asip.com>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] dynlink: set errno in case mprotect() fails
On Tue, Apr 21, 2026 at 08:34:11AM -0400, Rich Felker wrote:
> On Tue, Apr 21, 2026 at 09:57:27AM +0200, Florian Schmaus wrote:
> > The format string of the error message uses %m to print the value of
> > errno in human-readable form. But the low-level __syscall() macro,
> > used to invoke mprotect(), will not implicitly set errno. Fix this by
> > setting errno explicitly before emitting the error message.
> > ---
> > ldso/dynlink.c | 1 +
> > 1 file changed, 1 insertion(+)
> >
> > diff --git a/ldso/dynlink.c b/ldso/dynlink.c
> > index 715948f4f8b3..db0f9f52461c 100644
> > --- a/ldso/dynlink.c
> > +++ b/ldso/dynlink.c
> > @@ -1424,6 +1424,7 @@ static void reloc_all(struct dso *p)
> > long ret = __syscall(SYS_mprotect, laddr(p, p->relro_start),
> > p->relro_end-p->relro_start, PROT_READ);
> > if (ret != 0 && ret != -ENOSYS) {
> > + errno = -ret;
> > error("Error relocating %s: RELRO protection failed: %m",
> > p->name);
> > if (runtime) longjmp(*rtld_fail, 1);
> > --
> > 2.53.0
>
> I think this requires further analysis. See commit
> 63c67053a3e42e9dff788de432f82ff07d4d772a. Avoiding accessing errno was
> intentional here, and the access buried in the error path is probably
> a bug. But we need to look back at the constraints to determine how to
> fix it right.
At the early stage where there is no errno, error points to error_noop
so there is no problem except for lack of message. This is only for
libc itself and probably only happens if it was mislinked or if
there's bad seccomp policy.
However the case you're looking at is for other libraries after stage
2 has finished, and indeed there the message lacks a meaningful errno.
We could probably move the assignment into errno into the error()
function, by passing an error code to replace errno as the first
argument before the format string.
The other approach would be making a way to lookup strerror directly
here and use that, but calling libc functions is not valid yet at this
point. In practice it would probably happen to work, but that depends
on strerror not using errno/TLS and is not correct in our model of
what's legal to call at stage 2.
Rich
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