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Re: [UNIKRAFT PATCH] lib/syscall_shim: Force evaluation order of syscall arguments

Hi Felix,

thanks a lot for your work! I remember that we had once an issue of swapped argument orders but we did not figure out what caused it. Your reasoning makes sense to me and the patch looks good. We will take it upstream.



Reviewed-by: Simon Kuenzer <simon.kuenzer@xxxxxxxxx>

On 02.11.20 18:10, Felix Moebius wrote:
The file gen_uk_syscall.awk is used to generate the file uk_syscall.c.
The generated code calls the syscall handlers like this:

uk_syscall_e_fstat(va_arg(arg, long), va_arg(arg, long))

This is a problem since the order in which these va_arg expressions are being
evaluated is unspecified. This leads to arguments being swapped, depending
on how gcc decides to evaluate this.

When printing to stdout from musl this leads to an endless loop in

This patch fixes this by introducing temporary variables to store the varargs
and then passing these variables to the syscall handlers, taking into account
the number of required arguments.

Signed-off-by: Felix Moebius <felix.moebius@xxxxxxxxx>
  lib/syscall_shim/gen_uk_syscall.awk | 13 ++++++++++---
  1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/lib/syscall_shim/gen_uk_syscall.awk 
index 70c4df4..dc1a379 100644
--- a/lib/syscall_shim/gen_uk_syscall.awk
+++ b/lib/syscall_shim/gen_uk_syscall.awk
@@ -7,6 +7,11 @@ BEGIN {
print "long uk_vsyscall(long nr, va_list arg)\n{"
        print "\t(void) arg;\n"
+       printf "\t__maybe_unused long "
+       for (i = 1; i < max_args; i++)
+               printf "a%d, ", i
+       printf "a%d;\n", max_args
        print "\tswitch (nr) {"
@@ -16,11 +21,13 @@ BEGIN {
        sys_name = "SYS_" name
        args_nr = $2 + 0
        printf "\tcase %s:\n", sys_name;
+       for (i = 1; i <= args_nr; i++)
+               printf "\t\ta%s = va_arg(arg, long);\n", i;
        printf "\t\treturn uk_syscall_e_%s(", name;
        for (i = 1; i < args_nr; i++)
-               printf("va_arg(arg, long), ")
+               printf "a%d, ", i;
        if (args_nr > 0)
-               printf("va_arg(arg, long)")
+               printf "a%d", args_nr;
@@ -30,4 +37,4 @@ END {
        printf "\t\terrno = -ENOSYS;\n"
        printf "\t\treturn -1;\n"
        printf "\t}\n}\n"
\ No newline at end of file



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