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authorAndy Lutomirski <luto@mit.edu>2011-07-13 09:24:16 -0400
committerH. Peter Anvin <hpa@linux.intel.com>2011-07-14 17:57:09 -0700
commit98eedc3a9dbf90cecb91093d2a7fa083942b7d13 (patch)
tree0ed9320faed2d62caea337b978d5216a7fea55a8 /Documentation/ABI/stable
parent574c44fa8fa6262ffd5939789ef51a6e98ed62d7 (diff)
downloadlinux-bcache-98eedc3a9dbf90cecb91093d2a7fa083942b7d13.tar.zst
Document the vDSO and add a reference parser
It turns out that parsing the vDSO is nontrivial if you don't already have an ELF dynamic loader around. So document it in Documentation/ABI and add a reference CC0-licenced parser. This code is dedicated to Go issue 1933: http://code.google.com/p/go/issues/detail?id=1933 Signed-off-by: Andy Lutomirski <luto@mit.edu> Link: http://lkml.kernel.org/r/a315a9514cd71bcf29436cc31e35aada21a5ff21.1310563276.git.luto@mit.edu Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
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+On some architectures, when the kernel loads any userspace program it
+maps an ELF DSO into that program's address space. This DSO is called
+the vDSO and it often contains useful and highly-optimized alternatives
+to real syscalls.
+
+These functions are called just like ordinary C function according to
+your platform's ABI. Call them from a sensible context. (For example,
+if you set CS on x86 to something strange, the vDSO functions are
+within their rights to crash.) In addition, if you pass a bad
+pointer to a vDSO function, you might get SIGSEGV instead of -EFAULT.
+
+To find the DSO, parse the auxiliary vector passed to the program's
+entry point. The AT_SYSINFO_EHDR entry will point to the vDSO.
+
+The vDSO uses symbol versioning; whenever you request a symbol from the
+vDSO, specify the version you are expecting.
+
+Programs that dynamically link to glibc will use the vDSO automatically.
+Otherwise, you can use the reference parser in Documentation/vDSO/parse_vdso.c.
+
+Unless otherwise noted, the set of symbols with any given version and the
+ABI of those symbols is considered stable. It may vary across architectures,
+though.
+
+(As of this writing, this ABI documentation as been confirmed for x86_64.
+ The maintainers of the other vDSO-using architectures should confirm
+ that it is correct for their architecture.) \ No newline at end of file