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Date: Sun, 9 Jan 2022 01:00:03 -0500
From: Rich Felker <dalias@...c.org>
To: "Minqiang Chen (ptpt52)" <ptpt52@...il.com>
Cc: musl@...ts.openwall.com
Subject: Re: BUG fix: mmap pass wrong offset to kernel
On Tue, Nov 16, 2021 at 11:18:17AM -0500, Rich Felker wrote:
> On Tue, Nov 16, 2021 at 11:56:57AM +0800, Minqiang Chen (ptpt52) wrote:
> > From 146066a9794b8e39c53337b71a8476b86e79e7d4 Mon Sep 17 00:00:00 2001
> > From: Chen Minqiang <ptpt52@...il.com>
> > Date: Mon, 16 Oct 2017 08:57:41 +0800
> > Subject: [PATCH] musl: fix mmap pass wrong offset to kernel
> >
> > on 32bit platform for example off_t x=0x8d9eb000, the x/4096 result
> > is 0xfff8d9eb, but the sys_mmap2() is expecting 0x8d9eb to be pass to
> >
> > this happens on 32bit platform or 64bit platform when
> > x > = 0x80000000 (32bit platform)
> > or
> > x > = 0x8000000000000000 (64bit platform)
> >
> > Signed-off-by: Chen Minqiang <ptpt52@...il.com>
> > ---
> > src/mman/mmap.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/src/mman/mmap.c b/src/mman/mmap.c
> > index eff88d82..f225cdbb 100644
> > --- a/src/mman/mmap.c
> > +++ b/src/mman/mmap.c
> > @@ -26,7 +26,7 @@ void *__mmap(void *start, size_t len, int prot, int flags, int fd, off_t off)
> > __vm_wait();
> > }
> > #ifdef SYS_mmap2
> > - ret = __syscall(SYS_mmap2, start, len, prot, flags, fd, off/UNIT);
> > + ret = __syscall(SYS_mmap2, start, len, prot, flags, fd, (unsigned long)off/UNIT);
> > #else
> > ret = __syscall(SYS_mmap, start, len, prot, flags, fd, off);
> > #endif
> > --
> > 2.17.1
>
> This patch is wrong and truncates offsets over 32-bit (drops all the
> high bits). There is a bug here, but it's just that UNIT has the wrong
> type. commit b5bbe797493ea732d4cac15619753c545ed392af introduced the
> regression by making UNIT have type unsigned long long. It should have
> a small signed type; just int is fine.
Following up on this again: there isn't actually a bug here. All valid
offsets to mmap are non-negative off_t values, so coercion to unsigned
long long does not alter the value. If the offset is negative, it was
already caught by the mask against OFF_MASK in the first if statement.
Rich
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